Add @Nullable annotations #8852

Job	Run time
Build Ballerina Lang	7m 56s
Build Ballerina Distribution	1h 14m 41s
Build Stdlib Level (1)	2m 49s
Build Stdlib Level (2)	4m 57s
Build Stdlib Level (3)	9m 56s
Build Stdlib Level (4)	16m 8s
Build Stdlib Level (5)	4m 24s
Build Stdlib Level (6)	10m 55s
Build Stdlib Level (7)	35m 36s
Build Stdlib Level (8)	1h 31m 35s
Build Stdlib Level (9)	5m 36s
	4h 24m 33s

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