Update observables.py

src/braket/circuits/observables.py

@@ -322,9 +322,9 @@ def __init__(self, observables: list[Observable]):
             f"{'@'.join([obs.ascii_symbols[0] for obs in unscaled_factors])}"
         )
         all_targets = [factor.targets for factor in unscaled_factors]
-        if all(targets is None for targets in all_targets):
-            merged_targets = None
-        elif all(targets is not None for targets in all_targets):
+        if not any(all_targets):
+            merged_targets = QubitSet()
+        elif all(all_targets):
             flat_targets = [qubit for target in all_targets for qubit in target]
             merged_targets = QubitSet(flat_targets)
             if len(merged_targets) != len(flat_targets):