Add @Nullable annotations #8849

Job	Run time
Build Ballerina Lang	8m 13s
Build Ballerina Distribution	1h 5m 10s
Build Stdlib Level (1)	2m 17s
Build Stdlib Level (2)	3m 58s
Build Stdlib Level (3)	4m 50s
Build Stdlib Level (4)	15m 17s
Build Stdlib Level (5)	3m 54s
Build Stdlib Level (6)	10m 46s
Build Stdlib Level (7)	33m 46s
Build Stdlib Level (8)	1h 25m 14s
Build Stdlib Level (9)	4m 19s
	3h 57m 44s

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