fix: scaling: Return if there are no measurements

it should not be the case because the measurements come from
successful results, but otherwise it'd throw an exception.

sbws/lib/scaling.py

@@ -14,8 +14,10 @@ def bw_filt(bw_measurements):
     It is the equivalent to Torflow's ``filt_sbw``.
     ``mu`` in this function is the equivalent to Torflow's ``sbw``.
     """
-    mu = 1
-    if bw_measurements:
+    # This should never be the case, as the measurements come from successful
+    # results.
+    if not bw_measurements:
+        return 1
     # Torflow is rounding to an integer, so is `bw_mean_from_results` in
     # `v3bwfile.py`
     mu = round(mean(bw_measurements))