set default empty dataframe for self._poi_nex_df in NHDNetworks

src/troute-network/troute/NHDNetwork.py

@@ -45,7 +45,7 @@ def __init__(
         self.output_parameters = output_parameters
         self.verbose = verbose
         self.showtiming = showtiming
-        self._poi_nex_dict = None
+        self._poi_nex_df = pd.DataFrame()
         self._nexus_dict = None
         if self.verbose:
             print("creating supernetwork connections set")