Do not insert an empty img tag via image dialog

[indigoxela]

Not new, but so far I never realized, that.

Steps to reproduce:

• Add an image via image dialog
• With that image active, open the dialog again
• Remove the image via "Remove" button inside dialog
• Hit the dialog's "Insert" button

This results in the image tag to end up like <img alt="" width="" height="" data-align="none"> - without src attribute. That's a bit odd. Actually, I'd rather expect that the img tag gets removed then.

[indigoxela]

If one just hits the "Insert" button in the dialog without selecting any image, there's currently also an empty img tag. Also with figure/caption or link around it.