fix: prevent error when nshots=None. In that case frequencies and probabilities are set to None

src/qibotn/backends/quimb.py

@@ -112,18 +112,24 @@ class QuimbBackend(QibotnBackend, NumpyBackend):
             circuit.to_qasm(), psi0=initial_state
         )
 
-        frequencies = Counter(circ_quimb.sample(nshots)) if nshots is not None else None
+        if nshots:
+            frequencies = Counter(circ_quimb.sample(nshots))
             main_frequencies = {
                 state: count
-            for state, count in frequencies.most_common(self.n_most_frequent_states)
+                for state, count in frequencies.most_common(
+                    self.n_most_frequent_states
+                )
             }
-        computational_states = [state for state in main_frequencies.keys()]
+            computational_states = list(main_frequencies.keys())
             amplitudes = {
                 state: circ_quimb.amplitude(state) for state in computational_states
             }
             measured_probabilities = {
                 state: abs(amplitude) ** 2 for state, amplitude in amplitudes.items()
             }
+        else:
+            frequencies = None
+            measured_probabilities = None
 
         statevector = circ_quimb.to_dense() if return_array else None
         return TensorNetworkResult(