Add @Nullable annotations #8817

Job	Run time
Build Ballerina Lang	8m 16s
Build Ballerina Distribution	1h 6m 8s
Build Stdlib Level (2)	4m 49s
Build Stdlib Level (1)	2m 45s
Build Stdlib Level (3)	9m 14s
Build Stdlib Level (4)	16m 3s
Build Stdlib Level (5)	3m 23s
Build Stdlib Level (6)	11m 25s
Build Stdlib Level (7)	35m 26s
Build Stdlib Level (8)	42m 35s
Build Stdlib Level (9)	4m 25s
	3h 24m 29s

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