fix(mappers): Respect nullability of leaf properties when flattening schema #4192

Job	Run time
Test on 3.7 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 15s
External Tests	2m 18s
Test on 3.8 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 24s
Test on 3.9 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 11s
Test on 3.10 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 19s
Test on 3.11 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 6s
Test on 3.7 (tests) / macos-latest / SQLAlchemy: 2.*	2m 22s
Test on 3.8 (tests) / macos-latest / SQLAlchemy: 2.*	1m 24s
Test on 3.9 (tests) / macos-latest / SQLAlchemy: 2.*	1m 58s
Test on 3.10 (tests) / macos-latest / SQLAlchemy: 2.*	4m 42s
Test on 3.11 (tests) / macos-latest / SQLAlchemy: 2.*	1m 27s
Test on 3.7 (tests) / windows-latest / SQLAlchemy: 2.*	3m 9s
Test on 3.8 (tests) / windows-latest / SQLAlchemy: 2.*	2m 24s
Test on 3.9 (tests) / windows-latest / SQLAlchemy: 2.*	2m 50s
Test on 3.10 (tests) / windows-latest / SQLAlchemy: 2.*	2m 38s
Test on 3.11 (tests) / windows-latest / SQLAlchemy: 2.*	3m 49s
Test on 3.11 (tests) / ubuntu-latest / SQLAlchemy: 1.*	1m 29s
Test on 3.10 (doctest) / ubuntu-latest / SQLAlchemy: 2.*	45s
Test on 3.8 (mypy) / ubuntu-latest / SQLAlchemy: 2.*	56s
Coverage	0s
	38m 26s

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