Fix reading of strat_props

layer "NA" would result in NaN

nlmod/read/geotop.py

@@ -42,7 +42,7 @@ def get_lithok_colors():
 
 def get_strat_props():
     fname = os.path.join(NLMOD_DATADIR, "geotop", "REF_GTP_STR_UNIT.csv")
-    df = pd.read_csv(fname)
+    df = pd.read_csv(fname, keep_default_na=False, na_values="")
     # rename the columns to previously used values
     # so existing nlmod-code will keep working
     df = df.rename(
@@ -122,10 +122,6 @@ def to_model_layers(
     if strat_props is None:
         strat_props = get_strat_props()
 
-    # stap 2 create layer for each stratigraphy unit (geo-eenheid)
-    if strat_props is None:
-        strat_props = get_strat_props()
-
     # get all strat-units in Dataset
     strat = geotop_ds["strat"].values
     units = np.unique(strat)