Do not insert promotion if public key is empty

[NejcZdovc]

Right now when we fetch promotions from the server we just insert them and only check if they are active. We should also check if public key is valid (meaning not an empty list) before inserting it.

[srirambv]

Verification passed on

Brave	1.10.81 Chromium: 81.0.4044.138 (Official Build) beta (64-bit)
Revision	8c6c7ba89cc9453625af54f11fd83179e23450fa-refs/branch-heads/4044@{# 999}
OS	Windows 10 OS Version 1809 (Build 17763.379)

• Verified test plan from Promotion guards brave-core#5485
• Verified promotion showed error when trying to claim

Verification passed on

Brave	1.10.80 Chromium: 81.0.4044.138 (Official Build) dev (64-bit)
Revision	8c6c7ba89cc9453625af54f11fd83179e23450fa-refs/branch-heads/4044@{#999}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#5485

• Verified promotion showed error when trying to claim
  
• Verified the promotion is cleared
• Verified status of the promotion did not change

Verification PASSED on macOS 10.15.5 x64 using the following build:

Brave | 1.10.86 Chromium: 83.0.4103.61 (Official Build) (64-bit)
-- | --
Revision | 94f915a8d7c408b09cc7352161ad592299f384d2-refs/branch-heads/4103@{#561}
OS | macOS Version 10.15.5 (Build 19F101)

• Verified the case outlined via Promotion guards brave-core#5485 without issues
• ensured that the promotion was cleared once OK was clicked on the Oops, something is wrong. Please try again later. error.
• ensured that the status not any unexpected values where changed under the promotion table via publisher_info_db