Allow empty XML style tags when saving

[jpeterson976]

Fixes #1654

I added a check that sets node.value = '' if it doesn't already exist, which prevents the saving error from occurring and lets the empty tag get removed when swapping to the JSON/Visual Editor and back.

[maufcost]

LGTM! This works pretty well as far as my tests go. Clap Clap!

I tested by:

• Creating empty <b></b>, <i></i>, and others and going back and forth between editors.
• Styling empty spaces (also in conjunction with other text tags e.g. text, textGroup, etc)

[zachberry]

LGTM!