Fixed issue where "Empty" and "Not Empty" filters raised NotImplementedError on SQLAlchemy relationship attributes

[whchi]

When using sqlalchemy as database with the filter feature's "Empty" and "Not Empty", it works well for column but not for the relation fields(HasMany, HasOne)

throws error

# is_not_null
NotImplementedError: <function is_not at 0x105ace200>
# is_null
NotImplementedError: <function is_ at 0x105ace200>

Because the operator of sqlalchemy only works on columns not relationship

# starlette_admin/contrib/sqla/helpers.py
OPERATORS ={
   ...
    "is_null": lambda f, v: f.is_(None),
    "is_not_null": lambda f, v: f.is_not(None),
}

This PR fix this issue

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9646fbe) 100.00% compared to head (c74fdc3) 100.00%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #394   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           72        72           
  Lines         5559      5579   +20     
=========================================
+ Hits          5559      5579   +20     

Files	Coverage Δ
starlette_admin/contrib/sqla/helpers.py	100.00% <100.00%> (ø)
tests/sqla/test_sync_engine.py	100.00% <100.00%> (ø)

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[jowilf]

Thank you for your contribution 🎉