feat(auth): add JSpecify Null annotations to Auth #4511

Job	Run time
filter	1m 11s
downstream-compatibility (spring-cloud-gcp)	13m 57s
downstream-compatibility (java-pubsub)	2m 48s
downstream-compatibility (java-bigtable)	9m 3s
downstream-compatibility (java-firestore)	3m 20s
conditional-required-check	4s
	30m 23s

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