test(android): ignore empty events in group message stream

[Mabolla]

Summary

Fixes a flaky Android group message stream test by ignoring empty-body events before asserting on streamed text messages.

The stream can emit non-text/system events with an empty body, so assuming the first emitted item is the "hi" text message can cause the assertion to fail nondeterministically.

This keeps the change scoped to the test and avoids adding timing-based sleeps.

Testing

• git diff --check passes.
• Local Android instrumentation testing was not available because no Android SDK/device is configured in the local environment.
• The Android Gradle project configuration was verified successfully with JDK 17 / Gradle 8.11.1.

Related to #3941.

Note

Filter empty events from group message stream in Android tests

Applies a filter { it.body.isNotEmpty() } to the group.streamMessages() flow in GroupTest.kt so test assertions only operate on non-empty messages. This prevents spurious failures caused by empty events emitted before the actual message arrives.

^{Macroscope summarized 3246147.}