fix: Only send postMessage through worker if it's defined

[andrewnester]

Issue #, if available:

Description of changes:
Brought back from c9

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[codecov]

Codecov Report

Merging #4722 (d096fd9) into master (bf2c7e2) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4722      +/-   ##
==========================================
+ Coverage   71.19%   71.24%   +0.05%     
==========================================
  Files         553      553              
  Lines       55598    55598              
  Branches    10392    10393       +1     
==========================================
+ Hits        39584    39612      +28     
+ Misses      16014    15986      -28     

Flag	Coverage Δ
unittests	71.24% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/ace/worker/worker_client.js	48.97% <100.00%> (ø)
lib/ace/editor.js	81.61% <0.00%> (+1.97%)	⬆️

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