Make UpdateUserProfile consistent on clearing fields on Android

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

On iOS and Desktop, setting photo_url to nullptr resulted in no change to the user, and empty string resulted in clearing the field on user. On Android, both resulted in no change in the user. Make it consistently clear the field with an empty string, and do no change with nullptr. And added tests to verify.

---

Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

---

Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

---

Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

Integration test with FLAKINESS (succeeded after retry)

Requested by @a-maurice on commit d8ffa49
Last updated: Tue Oct 8 17:22 PDT 2024
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [Android] [1/3 os: ubuntu] [1/2 android_device: android_target] (1 failed tests)   CRASH/TIMEOUT [TEST] [FLAKINESS] [Android] [1/3 os: ubuntu] [1/2 android_device: emulator_ftl_target] (1 failed tests)   AggregateCountTest.TestCanQueryByDocumentIdUsingRefs [TEST] [FLAKINESS] [Android] [1/3 os: windows] [1/2 android_device: emulator_ftl_target] (1 failed tests)   CRASH/TIMEOUT

Add flaky tests to go/fpl-cpp-flake-tracker