fix($http): apply `transformResponse` even when `data` is empty

Note, that (as a by-product of the previous implementation) only non-empty
data was passed through the `transformResponse` pipeline. This is no
longer the case.

When using a custom `transformResponse` function, one should make sure it
can also handle an empty (i.e. falsy) `data` argument appropriately.

Fixes #12976
Closes #12979