VertexAI: make FunctionCallPart.args nullable

[tanzimfh]

FunctionCall.args is an optional field in the proto.

Requires google-gemini/deprecated-generative-ai-android#207 to be released.

[github-actions]

Release note changes

The following release notes were modified. Please ensure they look correct.

Release Notes

firebase-vertexai

### {{firebase_vertexai}} version 16.0.0-beta04 {: #vertex-ai_v16-0-0-beta04}

* {{feature}} added support for `responseSchema` in `GenerationConfig`.

* {{changed}} Made `FunctionCallPart.args` nullable.

[github-actions]

Unit Test Results

0 files   -     4  0 suites   - 4   0s ⏱️ -3s
0 tests  -   50  0 ✔️  -   50  0 💤 ±0  0 ❌ ±0 
0 runs   - 100  0 ✔️  - 100  0 💤 ±0  0 ❌ ±0 

Results for commit 4823bbf. ± Comparison against base commit 9017c35.

♻️ This comment has been updated with latest results.

[google-oss-bot]

Size Report ¹

Affected Products

• firebase-vertexai

  Type	Base (a41af7d)	Merge (4d609dd)	Diff
  aar	161 kB	161 kB	-53 B (-0.0%)
  apk (aggressive)	1.32 MB	1.32 MB	+112 B (+0.0%)
  apk (release)	9.32 MB	9.32 MB	+104 B (+0.0%)

Test Logs

• Base (a41af7d): https://github.com/firebase/firebase-android-sdk/actions/runs/9961008948
• Merge (4d609dd): https://github.com/firebase/firebase-android-sdk/actions/runs/9967351493

1. https://storage.googleapis.com/firebase-sdk-metric-reports/BEjN5fuaET.html

[google-oss-bot]

Coverage Report ¹

Affected Products

No changes between base commit (9017c35) and merge commit (88bd7cf).

Test Logs

• Merge (88bd7cf): https://github.com/firebase/firebase-android-sdk/actions/runs/10359606377

1. https://storage.googleapis.com/firebase-sdk-metric-reports/KDVtnhSZMS.html

[google-oss-bot]

Startup Time Report ¹

The report is too large (122,624 chars) to be displayed on GitHub. Please check this report on GCS.

1. https://storage.googleapis.com/firebase-sdk-metric-reports/RTXreGhD87/index.html