Fix creating rooms when the default is set to v12

[S7evinK]

This should fix #3669
We'd potentially send an empty string to GenerateCreateContent, even if we set the correct roomVersion before.

Pull Request Checklist

• I have added Go unit tests or Complement integration tests for this PR or I have justified why this PR doesn't need tests
• Pull request includes a sign off below or I have already signed off privately

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 52.21%. Comparing base (12b7038) to head (5dcf908).

Files with missing lines	Patch %	Lines
clientapi/routing/createroom.go	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3670      +/-   ##
==========================================
- Coverage   52.24%   52.21%   -0.04%     
==========================================
  Files         524      524              
  Lines       48399    48399              
==========================================
- Hits        25288    25273      -15     
- Misses      19602    19616      +14     
- Partials     3509     3510       +1     

Flag	Coverage Δ
unittests	52.21% <0.00%> (-0.04%)	⬇️

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[S7evinK]

Any feelings about bumping the default room version to 12? @kegsay @devonh

[devonh]

Nice fix! Surprised we haven't run into this before.

[devonh]

Any feelings about bumping the default room version to 12? @kegsay @devonh

I don't think that would be an issue. I don't have much context on how stable Dendrite is with v12. That would be my only hesitation.

[bones-was-here]

Participation in v12 rooms has been solid so far, but we haven't created many.
In a certain case gomatrixserverlib tries to parse a domain out of the room ID #3671 so probably that should be fixed before changing the default.

This patch seems to work for allowing the default to be configured to 12.