Type inference for ARRAY_INSERT function is incorrect

The type inference is implemented in SqlLibraryOperators.arrayInsertReturnType.
Since the spec of the function says that "if the index is larger than the array, the array is padded with NULL values", it implies that the type of the result is always an array with nullable elements, irrespective of the type of the elements of the input array.
Currently the type inference preserves the element type in the result.