SQL function `split` should return an array of non-nullable elements

Currently, split returns the data type ArrayType(StringType) which means the resultant array can contain nullable elements. However I do not see any case where the array can contain nulls.

In the case where either the provided string or delimiter is NULL, the output will be a NULL array. In case of empty string or no chars between delemiters, the output array will contain empty strings but never NULLs. So I propose we change the return type of split to mark elements as non-null.