Ensure QuantumCircuit.metadata is always a dict (Qiskit#1761)

* Ensure QuantumCircuit.metadata is always a dict

This is for compatibility with Qiskit/qiskit#9849

* fix order

* fix typo :(

* revert object->object() change

* make backportable

qiskit_aer/backends/aerbackend.py

@@ -426,13 +426,13 @@ def _execute_circuits_job(self, circuits, noise_model, config, job_id="", format
         # Take metadata from headers of experiments to work around JSON serialization error
         metadata_list = []
         for idx, circ in enumerate(circuits):
+            metadata_list.append(circ.metadata)
+            # TODO: we test for True-like on purpose here to condition against both None and {},
+            # which allows us to support versions of Terra before and after QuantumCircuit.metadata
+            # accepts None as a valid value. This logic should be revisited after terra>=0.24.0 is
+            # required.
             if circ.metadata:
-                metadata = circ.metadata
-                metadata_list.append(metadata)
-                circ.metadata = {}
-                circ.metadata["metadata_index"] = idx
-            else:
-                metadata_list.append(None)
+                circ.metadata = {"metadata_index": idx}
 
         # Run simulation
         aer_circuits = assemble_circuits(circuits)