Fix infinite loop when study search query consists of single "or"

[BasLee]

When entering 'or' in the study search field index page crashes due to an infinite loop.
This is because the search query parser interprets the 'or' as a separator between two search and-clauses (and no next search query can be found).
To fix this, the search query parser now interprets a single 'or' as a search term. This logic is also applied to search queries ending in a single or (e.g. 'tcga or').
Another solution would have been removing the trailing 'or', but this would also modify the search query shown in the search box, which is undesirable.

Testing

• tests added to QueryParser.spec.ts