Fix the issue missing text response of remotea2a agent when load previous session on adk web

[nguoithichkhampha]

Problem:
When I use a2a agent in adk by using RemoteA2aAgent. the message from a2a agent show normally on live session. But when I restart adk web and load the previous session, the message from a2a agent is disappeared

Root cause:
a2a agent send some event without content. It make adk web UI error in function combineA2uiDataParts

Solution:
Ignore the event without content or parts

Testing Plan

Manual End-to-End (E2E) Tests:
After this fix, I tried to run a new session and see the message from a2a agent after restart adk web

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[adk-bot]

Response from ADK Triaging Agent

Hello @nguoithichkhampha, thank you for creating this PR! It looks like you have done a great job describing the issue, root cause, and providing screenshots for the manual E2E tests.

However, after reviewing your pull request against our contribution guidelines, there are a couple of requirements that still need to be addressed:

1. Contributor License Agreement (CLA): The Google CLA check is currently failing. Please make sure to visit Google CLA to sign or link your agreement.
2. Unit Tests: To maintain code quality and prevent regressions, all code changes should include unit tests. Please add or update unit tests for your change under tests/unittests/ and include a summary of passed pytest results in the PR description.

Addressing these items will help our reviewers process your PR much more efficiently. Thank you for your contributions!