Replace array type hint with mixed[] type hint, since it means the same.
This way it's possible to require traversable type hint for new code.
Signed-off-by: Maurício Meneghini Fauth <mauricio@fauth.dev>
Since union type are now possible, using union with null makes more
clear that it is a union type.
Signed-off-by: Maurício Meneghini Fauth <mauricio@fauth.dev>