[NPUW] Fix Coverity null dereference (#33707)
### Details:
- Fix null dereference introduced in
https://github.com/openvinotoolkit/openvino/pull/33399
- Not a real problem, since `m_model_a_in_map` and `m_model_b_in_map`
will be empty thus `set_tensor` wouldn't be called, but adding check for
the saKe of the check.
### Tickets:
- E-199864
