ref(hc): Handle LazyObjects in serialize_generic_user

[RyanSkonnord]

Small tweak following #60065.

With the change in #60065, it might be safe not to do this in serialize_generic_user at all, but it's probably better to continue. Doing so by isinstance over hasattr (consistently with #60065) is safer.

[codecov]

Codecov Report

Merging #60120 (5f3efa8) into master (32aacd5) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #60120   +/-   ##
=======================================
  Coverage   80.85%   80.85%           
=======================================
  Files        5183     5183           
  Lines      227682   227683    +1     
  Branches    38309    38309           
=======================================
+ Hits       184093   184098    +5     
+ Misses      37968    37963    -5     
- Partials     5621     5622    +1     

Files	Coverage Δ
src/sentry/services/hybrid_cloud/user/serial.py	92.15% <100.00%> (+0.15%)	⬆️

... and 3 files with indirect coverage changes