Add @Nullable annotations #8780

Job	Run time
Build Ballerina Lang	7m 11s
Build Ballerina Distribution	1h 4m 7s
Build Stdlib Level (1)	2m 20s
Build Stdlib Level (2)	5m 43s
Build Stdlib Level (5)	2m 36s
Build Stdlib Level (3)	6m 28s
Build Stdlib Level (6)	11m 38s
Build Stdlib Level (7)	33m 6s
Build Stdlib Level (4)	15m 36s
Build Stdlib Level (9)	5m 10s
Build Stdlib Level (8)	38m 45s
	3h 12m 40s

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