Fix unique_short_code_plus_domain index in Microsoft SQL

[acelaya]

Closes #2319

This PR is part of the investigation for #2319, and includes:

1. A simplification of the query used when finding one short URL with fallback from domain to non-domain one. Basically the query run when a short URL is visited.
   Now this query does not try to join with the domains table if provided domain is already null.
2. A fix for the unique_short_code_plus_domain index in Microsoft SQL, so that it also applies to rows where domain_id is null.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.79%. Comparing base (259635e) to head (053e1f3).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #2363   +/-   ##
==========================================
  Coverage      93.78%   93.79%           
- Complexity      1705     1706    +1     
==========================================
  Files            277      277           
  Lines           5904     5912    +8     
==========================================
+ Hits            5537     5545    +8     
  Misses           367      367           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.