fix: do not fail when trying to read metadata of a missing geojson

Some geojson have been removed by mistake time ago (cf #1003), when someone
tries to load a map referencing them, it was until recently just
showing an error message, but since recently we try to get the
metadata from it, and this will crash.

umap/models.py

@@ -526,7 +526,10 @@ class DataLayer(NamedModel):
         metadata = self.settings
         if not metadata:
             # Fallback to file for old datalayers.
-            data = json.loads(self.geojson.read().decode())
+            try:
+                data = json.loads(self.geojson.read().decode())
+            except FileNotFoundError:
+                data = {}
             metadata = data.get("_umap_options")
             if not metadata:
                 metadata = {