Fix 'nuxeo-tag-suggestion' element when tag service does not create the provided tag

The tag service sanitizes the tag before adding it. In some cases, the sanitized tag comes up as an empty string (e.g. a tag containing blank spaces only), therefore does not get created. The problem is the 'nuxeo-tag-suggestion' element adds the un-sanitized tag anyway in its placeholder.

How to reproduce:

• navigate to a document/asset
• enter a single space in the Tags element (and press the + in the drop down to add it)

Actual result: the single-space tag is rendered in the list of tags associated to the document
Expected result: The single-space tag does not get added to the list of tags associated to the document as the *automation operation Services.TagDocument's response does not contain the single-space tag

Refreshing the page display the right list of tags, without the entered single-space tag.