Fix plugin source null reference exception

[NizhynTech]

Platforms affected

IOS

Motivation and Context

Description

cordova-plugin-advanced-websocket add/remove throw an error because source is null

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.21%. Comparing base (6a130fd) to head (06cd6a6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1630   +/-   ##
=======================================
  Coverage   80.21%   80.21%           
=======================================
  Files          18       18           
  Lines        5312     5312           
=======================================
  Hits         4261     4261           
  Misses       1051     1051           

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[dpogue]

This might need the same fix in the install path...

It might actually be better to fix this in cordova-common and ensure that all of the podspec values fall back to an empty array instead of being null/undefined: https://github.com/apache/cordova-common/blob/82c1b2f259f59423b3f59630142ea452cdf9be7a/src/PluginInfo/PluginInfo.js#L290-L298