Fix backspace in SimpleMDE

[zeripath]

The SimpleMDE backspace function refers to a trigger method of the textarea - which is not always present. This simple fix checks if the trigger is there before attempting to run it.

I'm not certain what is supposed to be being triggered here @blueworrybear do you have any more information?

Fix #9493

[codecov-io]

Codecov Report

Merging #9545 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9545      +/-   ##
==========================================
+ Coverage   42.07%   42.08%   +<.01%     
==========================================
  Files         574      574              
  Lines       75243    75243              
==========================================
+ Hits        31662    31665       +3     
  Misses      39667    39667              
+ Partials     3914     3911       -3

Impacted Files	Coverage Δ
models/unit.go	62.16% <0%> (-5.41%)	⬇️
models/webhook.go	69.39% <0%> (-1.07%)	⬇️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️
services/pull/check.go	50.7% <0%> (+2.11%)	⬆️
services/pull/temp_repo.go	34.18% <0%> (+2.56%)	⬆️

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[zeripath]

I've merged this but I'm not certain that the original reason why the trigger was there is actually covered.