Add @Nullable annotations #8806

Job	Run time
Build Ballerina Lang	7m 36s
Build Ballerina Distribution	1h 6m 11s
Build Stdlib Level (1)	2m 19s
Build Stdlib Level (2)	3m 32s
Build Stdlib Level (3)	5m 9s
Build Stdlib Level (4)	17m 12s
Build Stdlib Level (5)	2m 40s
Build Stdlib Level (6)	10m 27s
Build Stdlib Level (7)	33m 37s
Build Stdlib Level (8)	40m 33s
Build Stdlib Level (9)	4m 40s
	3h 13m 56s

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