mobile: more flow control improvements

[alyssawilk]

follow-ups on https://github.com/envoyproxy/envoy/pull/34211 correctly handling fin-with-no-payload as well.

Commit Message: Additional Description: Risk Level: Testing: Docs Changes: Release Notes:

Fixes https://github.com/envoyproxy/envoy/issues/34288

[repokitteh-read-only[bot]]

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:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/34303 was opened by alyssawilk.

see: more, trace.