Make default argument safer

For explanation see:
https://florimond.dev/blog/articles/2018/08/python-mutable-defaults-are-the-source-of-all-evil/

src/egon/data/importing/mastr.py

@@ -4,16 +4,19 @@
 import egon.data.config
 
 
-def download_mastr_data(data_stages=["cleaned"]):
+def download_mastr_data(data_stages=None):
     """
     Download MaStR data from Zenodo
 
     Parameters
     ----------
     data_stages: list
         Select data stages you want to download data for. Possible values:
-        'raw', 'cleaned'
+        'raw', 'cleaned'. Defaults to 'cleaned' if omitted.
     """
+    # Process inputs
+    if not data_stages:
+        data_stages = ["cleaned"]
 
     # Get parameters from config and set download URL
     data_config = egon.data.config.datasets()["mastr"]