Handle edge case where target doesn't have measurement defined

This commit fixes an edge case in the heurstic scoring where if a target
is present but doesn't have measurement defined we always return a score
of 0. In the case measure ment doesn't have measurement defined this
will fall back to looking at the degree of the qubit instead of trying
to use the readout error rate.

qiskit/transpiler/passes/layout/vf2_layout.py

@@ -211,9 +211,10 @@ def _score_layout(self, layout):
         on the chosen qubits. If BackendProperties are not available it uses the coupling map degree
         to weight against higher connectivity qubits."""
         bits = layout.get_physical_bits()
-        score = 0
+        score = None
         if self.target is not None:
             if "measure" in self.target:
+                score = 0
                 for bit in bits:
                     props = self.target["measure"].get((bit,))
                     if props is None or props.error is None:
@@ -223,7 +224,8 @@ def _score_layout(self, layout):
                         ) / len(self.coupling_map.graph)
                     else:
                         score += props.error
-        else:
+        if score is None:
+            score = 0
             if self.properties is None:
                 # Sum qubit degree for each qubit in chosen layout as really rough estimate of error
                 for bit in bits: