Make `instanceForProtocol:inContainer:` return `nullable T` #2248

Job	Run time
catalyst	3m 2s
pod-lib-lint (ios, macos-12)	7m 1s
pod-lib-lint (ios, macos-13)	12m 4s
pod-lib-lint (tvos, macos-12)	3m 42s
pod-lib-lint (tvos, macos-13)	4m 31s
quickstart	7m 50s
pod-lib-lint (macos --skip-tests, macos-12)	3m 51s
pod-lib-lint (macos --skip-tests, macos-13)	4m 9s
pod-lib-lint (watchos, macos-12)	3m 23s
pod-lib-lint (watchos, macos-13)	3m 10s
quickstart-ftl-cron-only	0s
spm (iOS, macos-12)	4m 33s
spm (iOS, macos-13)	8m 7s
spm (tvOS, macos-12)	4m 38s
spm (tvOS, macos-13)	3m 30s
spm (macOS, macos-12)	2m 51s
spm (macOS, macos-13)	2m 35s
spm (catalyst, macos-12)	2m 16s
spm (catalyst, macos-13)	2m 35s
spm (watchOS, macos-12)	4m 27s
spm (watchOS, macos-13)	6m 31s
abtesting-cron-only	0s
	1h 34m 46s

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