Fix missing handling of no-synthesis case in UnitarySynthesis (Qisk…

…it#9843)

Since moving `Optimize1qDecomposition` down to the Rust level, some of
its use in the default `UnitarySynthesis` plugin was allowing `None` to
be passed to an internal function unchecked.  This correctly returns the
"could not synthesise" value when appropriate.

qiskit/transpiler/passes/synthesis/unitary_synthesis.py

@@ -740,9 +740,10 @@ def run(self, unitary, **options):
 
         if unitary.shape == (2, 2):
             _decomposer1q = Optimize1qGatesDecomposition(basis_gates, target)
-            return _decomposer1q._gate_sequence_to_dag(
-                _decomposer1q._resynthesize_run(unitary, qubits[0])
-            )
+            sequence = _decomposer1q._resynthesize_run(unitary, qubits[0])
+            if sequence is None:
+                return None
+            return _decomposer1q._gate_sequence_to_dag(sequence)
         elif unitary.shape == (4, 4):
             # select synthesizers that can lower to the target
             if target is not None:

test/python/transpiler/test_unitary_synthesis.py

@@ -857,6 +857,12 @@ def __init__(self):
         result_qc = dag_to_circuit(result_dag)
         self.assertEqual(result_qc, QuantumCircuit(2))
 
+    def test_default_does_not_fail_on_no_syntheses(self):
+        qc = QuantumCircuit(1)
+        qc.unitary(np.eye(2), [0])
+        pass_ = UnitarySynthesis(["unknown", "gates"])
+        self.assertEqual(qc, pass_(qc))
+
 
 if __name__ == "__main__":
     unittest.main()