Fix nil pointer derefernce issues

[S7evinK]

Discovered while running https://gitlab.futo.org/load-testing/matrix-goose.

Dendrite locks up and runs into context cancelled, so the error is not sql.ErrNoRows nor "default" (and definitely shouldn't return that the account exists in this case)

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.49%. Comparing base (46902e5) to head (be13f7a).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...oomserver/storage/postgres/user_room_keys_table.go	0.00%	1 Missing and 1 partial ⚠️
roomserver/storage/sqlite3/user_room_keys_table.go	0.00%	1 Missing and 1 partial ⚠️
userapi/internal/user_api.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3379      +/-   ##
==========================================
- Coverage   67.51%   67.49%   -0.02%     
==========================================
  Files         521      521              
  Lines       47366    47371       +5     
==========================================
- Hits        31977    31975       -2     
- Misses      11405    11410       +5     
- Partials     3984     3986       +2     

Flag	Coverage Δ
unittests	52.70% <16.66%> (-0.05%)	⬇️

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