Do not emit null when serializing a nullable union type, but accept it if encountered in the allowedValues constraint

[jeskew]

Resolves #11544

Microsoft Reviewers: Open in CodeFlow

[github-actions]

Test Results (win-x64)

       33 files  ±0         33 suites  ±0   34m 54s ⏱️ +52s
10 417 tests +2  10 381 ✔️ +2  36 💤 ±0  0 ❌ ±0 
12 633 runs  +2  12 597 ✔️ +2  36 💤 ±0  0 ❌ ±0 

Results for commit 343ce32. ± Comparison against base commit c653d20.

[github-actions]

Test Results (linux-x64)

       33 files  ±0         33 suites  ±0   39m 22s ⏱️ + 6m 53s
10 405 tests +2  10 369 ✔️ +2  36 💤 ±0  0 ❌ ±0 
12 622 runs  +2  12 586 ✔️ +2  36 💤 ±0  0 ❌ ±0 

Results for commit 343ce32. ± Comparison against base commit c653d20.

[github-actions]

Test Results (linux-musl-x64)

       33 files  ±0         33 suites  ±0   35m 29s ⏱️ + 1m 27s
10 405 tests +2  10 369 ✔️ +2  36 💤 ±0  0 ❌ ±0 
12 622 runs  +2  12 586 ✔️ +2  36 💤 ±0  0 ❌ ±0 

Results for commit 343ce32. ± Comparison against base commit c653d20.

[github-actions]

Test Results (osx-x64)

       33 files  ±0         33 suites  ±0   1h 35m 21s ⏱️ -5s
10 409 tests +2  10 373 ✔️ +3  36 💤 ±0  0 ❌  - 1 
12 626 runs  +2  12 590 ✔️ +3  36 💤 ±0  0 ❌  - 1 

Results for commit 343ce32. ± Comparison against base commit c653d20.

[shenglol]