fixes test falling

Author: ankitkhushwaha

stingray/lightcurve.py

@@ -278,9 +278,7 @@ def __init__(
         if not skip_checks:
             time, counts, err = self.initial_optional_checks(time, counts, err, gti=gti)
 
-        if err_dist is None:
-            pass
-        elif isinstance(err_dist, str) or err_dist.lower() not in valid_statistics:
+        if err_dist is None or err_dist.lower() not in valid_statistics:
             # err_dist set can be increased with other statistics
             raise StingrayError(
                 "Statistic not recognized." "Please select one of these: ",