Add QuerySet.first_or_none

[dekoza]

As I mentioned in #1136 - here's the code providing missing first_or_none method.

I also update docstrings to better match what each method actually raises.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (5494503) to head (fc288bb).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1137   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          204       204           
  Lines        14835     14845   +10     
=========================================
+ Hits         14835     14845   +10     

Files	Coverage Δ
ormar/queryset/queryset.py	100.00% <100.00%> (ø)
tests/test_model_definition/test_models.py	100.00% <100.00%> (ø)

[collerek]

Sorry for the long time of review :/

[codspeed-hq]

CodSpeed Performance Report

Merging #1137 will improve performances by 13.67%

_{Comparing dekoza:feature/first_or_none (fc288bb) with master (5494503)}

Summary

⚡ 3 improvements
✅ 81 untouched benchmarks

Benchmarks breakdown

	Benchmark	master	dekoza:feature/first_or_none	Change
⚡	test_deleting_individually[20]	18.6 ms	16.4 ms	+13.67%
⚡	test_deleting_individually[40]	36.4 ms	32.1 ms	+13.57%
⚡	test_updating_models_individually[40]	47.4 ms	43 ms	+10.39%