fix(mappers): Respect nullability of leaf properties when flattening schema #4194

Job	Run time
Test on 3.7 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 15s
Test on 3.8 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 6s
Test on 3.9 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 17s
Test on 3.10 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 22s
Test on 3.11 (tests) / ubuntu-latest / SQLAlchemy: 2.*	1m 43s
Test on 3.7 (tests) / macos-latest / SQLAlchemy: 2.*	4m 44s
External Tests	2m 24s
Test on 3.8 (tests) / macos-latest / SQLAlchemy: 2.*	2m 37s
Test on 3.9 (tests) / macos-latest / SQLAlchemy: 2.*	1m 54s
Test on 3.10 (tests) / macos-latest / SQLAlchemy: 2.*	2m 5s
Test on 3.11 (tests) / macos-latest / SQLAlchemy: 2.*	2m 3s
Test on 3.7 (tests) / windows-latest / SQLAlchemy: 2.*	5m 38s
Test on 3.8 (tests) / windows-latest / SQLAlchemy: 2.*	3m 2s
Test on 3.9 (tests) / windows-latest / SQLAlchemy: 2.*	2m 20s
Test on 3.10 (tests) / windows-latest / SQLAlchemy: 2.*	2m 23s
Test on 3.11 (tests) / windows-latest / SQLAlchemy: 2.*	3m 1s
Test on 3.11 (tests) / ubuntu-latest / SQLAlchemy: 1.*	1m 39s
Test on 3.10 (doctest) / ubuntu-latest / SQLAlchemy: 2.*	51s
Test on 3.8 (mypy) / ubuntu-latest / SQLAlchemy: 2.*	1m 20s
Coverage	37s
	43m 21s

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