unittest: fix test for null tx input

Update the unittest that is meant to catch a transaction that is invalid
because it has a null input. The old test failed not because of that
but because it was considered a coinbase with too large script. This is
already checked with a different test, though.

The new test is *not* a coinbase since it has two inputs, but one of
them is null. This really checks the corresponding code path in
CheckTransaction.