fix(ui): fix MessageInputTheme.linkHighlightColor returning null for default theme.

[xsahil03x]

Fixes: #1546

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (13dce4d) 58.48% compared to head (7382cdb) 58.49%.

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1549   +/-   ##
========================================
  Coverage    58.48%   58.49%           
========================================
  Files          290      290           
  Lines        17653    17654    +1     
========================================
+ Hits         10325    10326    +1     
  Misses        7328     7328           

Impacted Files	Coverage Δ
...hat_flutter/lib/src/theme/message_input_theme.dart	79.66% <100.00%> (+0.17%)	⬆️

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