Stop sending empty data frame when input stream ends but the request stream is not ending.

[TingDaoK]

Issue #, if available:

• It's not against the spec to send out empty data frame, but if the data frame is empty and without the end of stream flag, it doesn't contain any useful information.
• We have the code try to prevent sending them
• However, when the input stream is ending, we fall into a situation that we still send the empty data frame.
• I consider it as an unexpected behavior.
• Also, some server doesn't support this empty data frame without end of stream, eg: httpbin.org will return GOAWAY with frame size error.
• Node, http2 also ignores the empty data frame

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.52%. Comparing base (fcc4932) to head (ebb8363).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #520      +/-   ##
==========================================
+ Coverage   79.50%   79.52%   +0.01%     
==========================================
  Files          27       27              
  Lines       11703    11703              
==========================================
+ Hits         9305     9307       +2     
+ Misses       2398     2396       -2     

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