add default values

app.py

@@ -83,8 +83,12 @@ def predict(mseeds=[], waveforms="", stations=""):
 
         picks.extend(tmp_picks)
 
-    picks_df = pd.DataFrame(picks)
-    picks_df.to_csv("picks.csv", index=False)
+    if len(picks) > 0
+        picks_df = pd.DataFrame(picks)
+        picks_df.to_csv("picks.csv", index=False)
+    else:
+        picks_df = None
+        os.system("touch picks.csv")
 
     return picks_df, "picks.csv", json.dumps(picks)