* Fix build filter coupling map with mix ideal/physical targets

This commit fixes a small issue in the Target.build_coupling_map method
with filter_idle_qubits=True. Previously the internal method used to
filter the idle qubits from the coupling map's creation was not
accounting for the case that an instruction in the target could be
ideal. This was never caught because normally if you have an ideal fixed
width instruction in the target that is modelling a simulator which
doesn't have any connectivity constraints and the function would never
be called. But in targets that have a mix of both ideal globally
defined and physical gates the filter code couldn't handle the None used
to represent an ideal globally available gate. This commit fixes that
handling to just ignore the global gate like the normal coupling map
construction does.

* Fix lint

(cherry picked from commit 4dfcba2)