fixes balance being an empty string

[AndriusA]

Resolves brave/brave-browser#10315

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

Steps to reproduce issue (Verified on Brave Dev 1.11.82 on Macos):

1. Start with a fresh profile
2. Verify wallet with uphold, there should not be any grants in the wallet already
3. As soon as wallet is verified, logs should contain POST requests to v2/wallet/WALLET_ID/claim endpoint including "denomination":{"amount":"","currency":"BAT"} in the body and server responses with error code 500

For testing, browser was launched with:

--args --user-data-dir=./profile-5883 \
  --enable-logging=stderr \
  --vmodule="*/bat-native-ledger/*"=6,"*/brave_rewards/*"=6,"*/bat-native-ads/*"=6,"*/bat-native-confirmations/*"=6,"*/brave_ads/*"=6 \
  --rewards=staging=true

And relevant output:

[ REQUEST ]
> URL: https://ledger-staging.mercury.basicattentiontoken.org/v2/wallet/ZZZZ/claim
> Method: UrlMethod::POST
> Content: {"anonymousAddress":"ZZZZ","signedTx":{"body":{"denomination":{"amount":"","currency":"BAT"},"destination":"ZZZZ"},"headers":{"digest":"SHA-256=4GobaOVP+I+43Eh1dOOUaKiNzFN0E0gB0aY48Y0rhWw=","signature":"keyId=\"primary\",algorithm=\"ed25519\",headers=\"digest\",signature=\"oxsmsZbsi4uM1cBKIJXX2dfomXzs733LoH8rgltfgiUvZnjdFY8mTzDnDVR6JNyNbo2/KnDDtshG5pnujdJgBg==\""},"octets":"{\"denomination\":{\"amount\":\"\",\"currency\":\"BAT\"},\"destination\":\"ZZZZ\"}"}}
> Content Type: application/json; charset=utf-8
> Header Content-Type: application/json; charset=UTF-8
[95374:775:0630/132704.611833:VERBOSE6:wallet.cc(190)]
[ RESPONSE - OnTransferAnonToExternalWallet ]
> Url: https://ledger-staging.mercury.basicattentiontoken.org/v2/wallet/ZZZZ/claim
> Result: Failure
> HTTP Code: 500
> Body: {"statusCode":500,"error":"Internal Server Error","message":"An internal server error occurred"}

With the issue fixed and calls to v2/wallet/WALLET_ID/claim should not be made or amount in the POST payload should be a non-empty numeric string

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[emerick]

LGTM, a couple of minor comments

[kjozwiak]

Verification PASSED on macOS 10.15.5 x64 using the following build:

Brave | 1.12.72 Chromium: 84.0.4147.68 (Official Build) nightly (64-bit)
-- | --
Revision | e7b7234037639b407e69c5428c891ce018cd6c0d-refs/branch-heads/4147@{#772}
OS | macOS Version 10.15.5 (Build 19F101)

• Created a new account on staging and waited till Uphold verified the account
• Once verified, ensure that the amount in the POST request was a non-empty numeric string

Example:

[ REQUEST ]
> URL: https://ledger-staging.mercury.basicattentiontoken.org/v2/wallet/33df2720-d774-4965-8bf8-c8b584e4e305/claim
> Method: UrlMethod::POST
> Content: {"anonymousAddress":"5672d309-f9b2-420b-82e8-74e04a67ecba","signedTx":{"body":{"denomination":{"amount":"0","currency":"BAT"},"destination":"d587539b-a32f-413d-80e9-fd1e1ca8b4dc"},"headers":{"digest":"SHA-256=icQV0kJzJUsmPehFR2qDVKh6QLLv1Y1g2NyjeBblrvU=","signature":"keyId=\"primary\",algorithm=\"ed25519\",headers=\"digest\",signature=\"yPhHvjHBgmndZcYtc+waJlndc2gkeX45bFlR5Nb3dkhA74S1CM0b9axFmddxy1qWw7GHHAwBeN/KIWvJqJpCCg==\""},"octets":"{\"denomination\":{\"amount\":\"0\",\"currency\":\"BAT\"},\"destination\":\"d587539b-a32f-413d-80e9-fd1e1ca8b4dc\"}"}}
> Content Type: application/json; charset=utf-8
> Header Content-Type: application/json; charset=UTF-8