Guard against empty tags

[nelsonkopliku]

Currently our tagging system allows empty tags.
If I press enter on a tag input without typing any text, trento allows that tag to be created.

I would argue about the usefulness of an empty tag already here.

Then this opens the issue of not being able to delete those tags.
When I refresh the page I get the items with the empty tag pill, and when I try to delete it, the backend goes 404 because of a non-existent deletion endpoint.

Simply put

the deletion endpoint should be something like

DELETE /api/clusters/6bd7ec60-8cb1-5c6b-a892-29e1fd2f8380/tags/<the-tag-value>

When <the-tag-value> is empty, the computed enpoint being called is

DELETE /api/clusters/6bd7ec60-8cb1-5c6b-a892-29e1fd2f8380/tags/

which does not exist.

[nelsonkopliku]

The backend part has been addressed here #364

[arbulu89]

Fix for frontend: #390