Fix panic if unable to assign a state key NID

[S7evinK]

Main change is the added error check before storing the eventStateKeyNID in the cache.

The panic happens if we attempt to set the cache for 0 multiple times, because the cache is immutable.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (9a5a567) 65.76% compared to head (61816c1) 65.71%.

Files	Patch %	Lines
roomserver/storage/shared/storage.go	40.00%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3294      +/-   ##
==========================================
- Coverage   65.76%   65.71%   -0.06%     
==========================================
  Files         509      509              
  Lines       57461    57464       +3     
==========================================
- Hits        37790    37763      -27     
- Misses      15842    15865      +23     
- Partials     3829     3836       +7     

Flag	Coverage Δ
unittests	49.74% <40.00%> (-0.09%)	⬇️

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