unset categories can't be returned by analyse

**Describe the bug**
When there is already a category 'unset' string that is the same or contains any character within another string, the analyse endpoint will not color by category.

**To Reproduce**
Steps to reproduce the behavior:
1. In the highlighter, select just the letter "p" and leave it **uncategorised**.
2. Highlight another word that has "p" within it (e.g "leap") and **categorize** it.
3. Go to 'TakeTwo web api' text creator page and type in the second word you highlighted ("leap").
4. See that the category color does not show.

**Expected behavior**
Should highlight all matches, for example both "p" and "pappa".

**Additional context**
We can fix this by filtering out the "unset" categories when using the analyse endpoint. TO DO.