[minor] bug on old run script default values

Without the default value specified for `click` option, `None` will be used.

src/scripts/run_network.py

@@ -196,6 +196,7 @@ def mpi_excepthook(type, value, traceback):
 @click.option(
     "--coordinates-namespace",
     required=False,
+    default="Coordinates",
     type=str,
     help="namespace for coordinates when cells without morphologies are instantiated",
 )