Steps to replicate the issue (include links if applicable):

• Pick a statement to edit on mex
• select add qualifier or reference on that statement
• pick a property, and as a value, select unknown or no value options on snak type
  • if the user enters any text into the field, then switches to unknown or no value, the add button works
• try to publish

What happens?:

The add button stays disabled and no error is shown - is is impossible to publish

What should have happened instead?:

The button becomes active and we can publish the "unknown" or "no value" qualifier or statement, or an error is displayed

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):
laptop - firefox - mobile desktop - beta / prod

A/C

• users can add unknown and no value qualifers and references without having entered a value
• tests updated as needed