PTV-1911: Fix user search bugs re underscores + mongo errors

[MrCreosote]

• Fixed a bug where if a user name contained an underscore and the search term included the underscore followed by at least one letter the search wouldn't match
• Fixed a bug where if the search terms were all illegal characters such that the search list was empty for a requested search a mongo error would be thrown

Fixes #435

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.36364% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.37%. Comparing base (b93fdd8) to head (5527329).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #439 +/- ## ============================================= - Coverage 93.37% 93.37% -0.01% - Complexity 2144 2151 +7 ============================================= Files 126 126 Lines 7531 7558 +27 Branches 1178 1184 +6 ============================================= + Hits 7032 7057 +25 - Misses 456 458 +2 Partials 43 43 ```