update logic becuase the waterbody_types_df_sub may be empty for sims…

… without waterbodies" (#538)

Co-authored-by: awlostowski-noaa <[email protected]>

src/troute-routing/troute/routing/compute.py

@@ -166,7 +166,8 @@ def _prep_reservoir_da_dataframes(reservoir_usgs_df, reservoir_usace_df, waterbo
     else:
         reservoir_usgs_df_sub = pd.DataFrame()
         reservoir_usgs_df_time = pd.DataFrame().to_numpy().reshape(0,)
-        waterbody_types_df_sub.loc[waterbody_types_df_sub['reservoir_type'] == 2] = 1
+        if not waterbody_types_df_sub.empty:
+            waterbody_types_df_sub.loc[waterbody_types_df_sub['reservoir_type'] == 2] = 1
 
     # select USACE reservoir DA data waterbodies in sub-domain
     if not reservoir_usace_df.empty:
@@ -178,7 +179,8 @@ def _prep_reservoir_da_dataframes(reservoir_usgs_df, reservoir_usace_df, waterbo
     else: 
         reservoir_usace_df_sub = pd.DataFrame()
         reservoir_usace_df_time = pd.DataFrame().to_numpy().reshape(0,)
-        waterbody_types_df_sub.loc[waterbody_types_df_sub['reservoir_type'] == 3] = 1
+        if not waterbody_types_df_sub.empty:
+            waterbody_types_df_sub.loc[waterbody_types_df_sub['reservoir_type'] == 3] = 1
 
     return reservoir_usgs_df_sub, reservoir_usgs_df_time, reservoir_usace_df_sub, reservoir_usace_df_time, waterbody_types_df_sub