[release/9.0] Update TreeView's TvnEndLabelEdit method to set label to null when no text has changed

[github-actions[bot]]

Backport of #13588 to release/9.0

Fixes https://github.com/dotnet/winforms/issues/13583

Proposed changes

Updates TvnEndLabelEdit to pass null for the new label when the text is unchanged, preventing exception scenarios on Enter key presses.

Customer Impact

• The user can use the Enter key to exit the edit state of the TreeNode when the text has not changed.

Regression?

• Yes, introduced in https://github.com/dotnet/winforms/pull/10149

Testing

• Manual testing with the user-provided project

Risk

• Low

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.85513%. Comparing base (615dd68) to head (cd68cf0).

Additional details and impacted files

@@                  Coverage Diff                  @@
##           release/9.0      #13614         +/-   ##
=====================================================
+ Coverage     74.85339%   74.85513%   +0.00173%     
=====================================================
  Files             3022        3022                 
  Lines           630570      630570                 
  Branches         46810       46810                 
=====================================================
+ Hits            472003      472014         +11     
+ Misses          155171      155167          -4     
+ Partials          3396        3389          -7     

Flag	Coverage Δ
Debug	74.85513% <75.00000%> (+0.00173%)	:arrow_up:
integration	18.02368% <0.00000%> (+0.00316%)	:arrow_up:
production	47.90503% <75.00000%> (+0.00386%)	:arrow_up:
test	96.99900% <ø> (ø)
unit	44.92984% <75.00000%> (-0.00105%)	:arrow_down:

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