fix(database, update): allow empty objects in ref.update()

[mikehardy]

Description

As pointed out by @daveGregorian firebase-js-sdk allows empty objects
while the code here was enforcing that the objects had at least one key

Related issues

Fixes #5218

Release Summary

Conventional commit message

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

Verified (with investigation in linked issue) that firebase-js-sdk allows empty objects through), added an e2e test that verifies firebase-ios-sdk and firebase-android-sdk handle empty objects, and those work, so removing the check in our javascript layer should be fine

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[codecov]

Codecov Report

Merging #5220 (f9b81f9) into master (589fcb0) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head f9b81f9 differs from pull request most recent head 269a3a8. Consider uploading reports for the commit 269a3a8 to get more accurate results

@@            Coverage Diff             @@
##           master    #5220      +/-   ##
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- Coverage   88.87%   88.86%   -0.00%     
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  Files         109      109              
  Lines        3744     3742       -2     
  Branches      360      360              
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- Hits         3327     3325       -2     
  Misses        370      370              
  Partials       47       47