Change lat, lon properties to return None

Slight code simplification, allowing get() to
return None when the field is missing. The related
if statement in Jinja2 accepts None as False.

Co-authored-by: Simon Conseil <contact@saimon.org>

src/sigal/gallery.py

@@ -289,12 +289,13 @@ class Image(Media):
 
     @cached_property
     def lat(self):
-        return self.markdown_metadata.get("lat", {})
+        """If not `None`, latitude extracted from the Markdown index.md file."""
+        return self.markdown_metadata.get("lat")
 
     @cached_property
     def lon(self):
-        """Other metadata extracted from the Markdown index.md file."""
-        return self.markdown_metadata.get("lon", {})
+        """If not `None`, longitude extracted from the Markdown index.md file."""
+        return self.markdown_metadata.get("lon")
 
     @cached_property
     def raw_exif(self):