AUI-1669 - Image Viewer Base throws error when no images or sources are provided

Hi Jon,

Attached is an update for https://issues.liferay.com/browse/AUI-1669. 

I moved the sources checks close to where the issue will occur as well as removed the sources variable caching changes.

When I went to deploy this in portal to make sure it fixed https://issues.liferay.com/browse/LPS-50278, I noticed it was already fixed in portal by https://issues.liferay.com/browse/LPS-48449.  But I still think a fix for this issue is needed on the alloy end as well.

Please let me know if there are any issues.

Thanks!