Enable the New Spine db button in freshly created Data stores

Add Data store dialog was returning an empty URL dict instead of
None which caused Data Store to disable the New Spine db button.
The dialog should return None as the URL so Data stores can
use their default initial URL which sets dialog to "sqlite"
enabling the button.

spine_items/data_store/data_store.py

@@ -58,7 +58,7 @@ def __init__(self, name, description, x, y, toolbox, project, url):
         except OSError:
             self._logger.msg_error.emit(f"[OSError] Creating directory {self.logs_dir} failed. Check permissions.")
         if url is None:
-            url = {}
+            url = {"dialect": "sqlite"}
         self._url = self.parse_url(url)
         self._url_validated = False
         self._resource_to_replace = None

spine_items/data_store/widgets/add_data_store_widget.py

@@ -37,7 +37,7 @@ def call_add_item(self):
                 "description": self.description,
                 "x": self._x,
                 "y": self._y,
-                "url": {},
+                "url": None,
             }
         }
         self._toolbox.add_project_items(item)