Disallow cast from other datatypes to NullType

[viirya]

Describe the bug

We allow casting from other datatypes to NullType (see can_cast_types). I suspect that this is a mistake.

For example, Spark doesn't allow casting from any datatypes to NullType.

I also checked C++ Cast kernels. Mostly the datatypes cannot cast to NullType (e.g. https://github.com/apache/arrow/blob/53752adc6b81166cd4ee7db5a819494042f29197/cpp/src/arrow/compute/kernels/scalar_cast_test.cc#L187-L211). I said "mostly" because I don't see explicit tests for other cases like Decimal, but I suspect that it is not allowed but just lack of test coverage.

I don't see there is cast kernel in C++ for casting to NullType except for a special case (dict<null> -> null): https://github.com/apache/arrow/blob/53752adc6b81166cd4ee7db5a819494042f29197/cpp/src/arrow/compute/kernels/scalar_cast_numeric.cc#L748-L751. Even for that, there is a comment questioning about it.

To Reproduce
Steps to reproduce the behavior:

Expected behavior

Cast kernel should not allow casting from any datatypes to NullType.

Additional context
Add any other context about the problem here.

[liukun4515]

The result of discussion: #1922 (comment)

[liukun4515]

I will give a follow-up pr to remove casting other data type to Null Data type.