Fix: updateDocTooltip with undefined this.completions

[mkslanc]

Issue #, if available: #5753

Description of changes:

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Pull Request Checklist:

• No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.10%. Comparing base (e237e54) to head (b78930f).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5754   +/-   ##
=======================================
  Coverage   87.10%   87.10%           
=======================================
  Files         602      602           
  Lines       44094    44094           
  Branches     7233     7235    +2     
=======================================
  Hits        38408    38408           
  Misses       5686     5686           

Flag	Coverage Δ
unittests	87.10% <100.00%> (ø)

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