provision for empty data frame in new workout

rowers/dataprep.py

@@ -853,6 +853,9 @@ def save_workout_database(f2, r, dosmooth=True, workouttype='rower',
                 powerperc=powerperc, powerzones=r.powerzones)
     row = rdata(f2, rower=rr)
 
+    if row.df.empty:
+        return (0, 'Error: CSV data file was empty')
+    
     dtavg = row.df['TimeStamp (sec)'].diff().mean()
 
     if dtavg < 1: