fix(mac): don't fail if the icon path given doesn't exist

[malept]

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• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Noticed in #1100. It generates a warning if the icon specified isn't found.

[codecov-io]

Codecov Report

Merging #1101 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1101   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          14      14           
  Lines         711     712    +1     
======================================
+ Hits          711     712    +1

Impacted Files	Coverage Δ
src/platform.js	100% <ø> (ø)	⬆️
src/mac.js	100% <100%> (ø)	⬆️

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