fix: plugin podspec with no config elements

[erisu]

Motivation and Context

Fail to install plugins with missing podspec config elements.

fixes #913

Description

Add conditional checks.

Testing

• npm t
• plugin add without podspec's config element.
• plugin add with podspec's config element.

Checklist

• I've run the tests to see all new and existing tests pass
• 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)

[codecov-io]

Codecov Report

Merging #1067 (80418c5) into master (fb5fd2f) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1067      +/-   ##
==========================================
+ Coverage   74.84%   74.88%   +0.04%     
==========================================
  Files          13       13              
  Lines        1721     1724       +3     
==========================================
+ Hits         1288     1291       +3     
  Misses        433      433              

Impacted Files	Coverage Δ
bin/templates/scripts/cordova/Api.js	71.83% <100.00%> (+0.30%)	⬆️

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[NiklasMerz]

LGTM thanks @erisu

[brodycj]

Thanks for fixing!