Filtering pull requests by username through REST API fails if database/query username casing doesn't match

Issue Summary

When searching pull requests using the REST API with a username filter, if a user's case-insensitive-but-case-preserving username in the database doesn't match the casing used in the query (e.g. username "bbs6100" in the database, and "BbS6100" used in the query), the query fails with a NullPointerException.

Steps to Reproduce

1. Create a user with a given name, like "bbs6100"
2. Search for pull requests using username.1=bbs6100 (matching the database's casing):

   curl -u <admin_user:password> -X GET -H "X-PrettyPrint:1" -H "Content-Type: application/json" <Bitbucket_base_url>/rest/api/1.0/projects/PR2/repos/myrepotest/pull-requests?username.1=bbs6100
   

   Retrieves pull requests successfully.

   {
     "size": 1,
     "limit": 25,
     "isLastPage": true,
     "values": [
       {
         "id": 1,
         "version": 1,
         "title": "first pull",
         "description": "pr",
   

3. Search for pull requests using username.1=BbS6100 (case mismatch):

   curl -u <admin_user:password> -X GET -H "X-PrettyPrint:1" -H "Content-Type: application/json" <Bitbucket_base_url>/rest/api/1.0/projects/PR2/repos/myrepotest/pull-requests?username.1=Bbs6100
   

   The request fails like this:

   {
     "errors": [
       {
         "context": null,
         "message": "An error occurred while processing the request. Check the server logs for more information.",
         "exceptionName": null
       }
     ]
   }
   

Expected Results

The REST API rest/api/1.0/projects/PR2/repos/myrepotest/pull-requests?username.1=Bbs6100 should work in a case-insensitive manner and both queries should return the same results.

Actual Results

The request fails with a NullPointerException, which is logged similar to the following in atlassian-bitbucket.log:

20-03-20 12:38:50,273 ERROR [http-nio-26100-exec-8] bbs6100 @Z10WHZx758x36x0 0:0:0:0:0:0:0:1 "GET /rest/api/1.0/projects/PR2/repos/myrepotest/pull-requests HTTP/1.1" c.a.s.i.r.e.DefaultUnhandledExceptionMapperHelper Unhandled exception while processing REST request: "GET /rest/api/1.0/projects/PR2/repos/myrepotest/pull-requests HTTP/1.1"
java.lang.NullPointerException: user

Workaround

Ensure the username used in the query matches the casing used in the database.