Don't write spec to podfile when it's an empty string

[piotr-cz]

Platforms affected

iOS

Motivation and Context

When plugin framework tag contains an empty spec value (spec=""), Podfile becomes malformed and pod install command fails with

ArgumentError - Illformed requirement `""`

Example:

    <framework src="Fabric" type="podspec" spec="~> 1.7.6"/>
    <framework src="Crashlytics" type="podspec" spec="~> 3.10.1"/>
    <framework src="Firebase/Core" type="podspec" spec=""/>

Creates

	pod 'Fabric', '~> 1.7.6'
	pod 'Crashlytics', '~> 3.10.1'
	pod 'Firebase/Core', ''

Proposed change checks for an empty spec value and doesn't write version number:

	pod 'Fabric', '~> 1.7.6'
	pod 'Crashlytics', '~> 3.10.1'
	pod 'Firebase/Core'

Description

I'm not sure if it's a bug in cordova-ios or specific plugin (cordova-plugin-firebase-crashlytics).
However cordova documentation for the framework tag doesn't specify if the spec attribute can be omitted so it seems valid to me to use empty value.

Testing

• Install cordova-plugin-firebase-crashlytics plugin in verbose mode:
  cordova plugin add cordova-plugin-firebase-crashlytics --verbose
• Installation doesn't fail

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-io]

Codecov Report

Merging #577 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #577   +/-   ##
=======================================
  Coverage   74.58%   74.58%           
=======================================
  Files          11       11           
  Lines        1826     1826           
=======================================
  Hits         1362     1362           
  Misses        464      464

Impacted Files	Coverage Δ
bin/templates/scripts/cordova/lib/Podfile.js	73.02% <100%> (ø)	⬆️

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[dpogue]

👍 from me

[dpogue]

Thanks for finding and fixing this! I'll merge it in for the upcoming 5.0.1 patch release :)

[piotr-cz]

After thinking about it a little more, it's supposed to be considered as bug in cordova-ios v5 and not in plugins as v4 has evaluating emptiness of the spec value before adding it to the Podfile