back in pre-postgresql-42* days we used to use -888 and -888.888 to represent int NULL and float NULL respectively, quite ...primitive but it worked. We should retain this old mapping , but once all systems are up to date we will start storing and reading NULLs in arrays as NULLs.
Using that mapping is not something that we would entertain.
If you are going to actually store NULL in the array, how would that work with primitives ?