BUG: core: result_type(0, np.timedelta64(4)) would seg. fault.

Before this change, the line in convert_datatype.c

    result = PyArray_CastDescrToDType(all_descriptors[0], common_dtype);

would trigger a seg. fault with a call such as

    np.result_type(0, np.timedelta64(4))

The problem was that `all_descriptors[0]` was NULL, and it was
dereferenced in PyArray_CastDescrToDType.

The code in the loop that followed the above line avoided passing
NULL values in all_descriptors[i] to PyArray_CastDescrToDType by
checking that the corresponding input argument to `result_type`
was not a Python scalar.  A different code path was taken in that
case.  The seg. fault is fixed by applying that same check to the
first argument.

Closes gh-20077.

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