fix(GH-1432): Default content src when content tag is missing

[breautek]

Platforms affected

Android

Motivation and Context

fixes #1432

If the project is missing the <content> tag inside their config.xml, Cordova will completely crash with a fatal error.

Description

Inside ConfigXMLParser, I added a onPostParse private method in which I check to see if the contentSrc string is still null. If so, it can be assumed that the <content> tag was completely missing and the contentSrc string is then fallen back to the default content source, which is now powered by a constant.

Testing

Ran npm test and manually ran against a project with a missing <content>. Observing the crash in cordova-android@11, and observing the fix using locally packed tarball install.

Additionally I also tested renaming the index.html and setting <content src> respectfully to confirm no breakage.

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

Merging #1573 (2e293b4) into master (04723eb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1573   +/-   ##
=======================================
  Coverage   72.13%   72.13%           
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  Files          21       21           
  Lines        1751     1751           
=======================================
  Hits         1263     1263           
  Misses        488      488           

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