Allow null as arguments type

[SergeAstapov]

When using {{svg-jar}} helper, optional named arguments need to be null (so they can be filtered out in via isNone in https://github.com/ghedamat/ember-svg-jar/blob/v2.4.6/packages/ember-svg-jar/addon/utils/make-svg.js#L107).

If argument value is undefined, it gets added to DOM, which may cause issues with arguments like width and height.

This PR fixes types to add null as allowed arg type.

[SergeAstapov]

@evoactivity would you be able to land and release "this as is"? CI seem unhappy due to There appears to be trouble with your network connection.

[evoactivity]

Yep, I'll be creating a release for this tomorrow.

[evoactivity]

I pushed the release for this just now. Looks like I'll be tasking ownership on the repo, I'm going to setup changesets so people with merge access will be able to release directly from the repo.