Fix git DataSource clone authentication

Anonymous git clones (in GitLab) require the username and password not
to be set in order to successfully clone. This patch will define clone
args only, if the username passed is not empty.

netbox/core/data_backends.py

@@ -103,12 +103,13 @@ def fetch(self):
         }
 
         if self.url_scheme in ('http', 'https'):
-            clone_args.update(
-                {
-                    "username": self.params.get('username'),
-                    "password": self.params.get('password'),
-                }
-            )
+            if self.params.get('username'):
+                clone_args.update(
+                    {
+                        "username": self.params.get('username'),
+                        "password": self.params.get('password'),
+                    }
+                )
 
         if settings.HTTP_PROXIES and self.url_scheme in ('http', 'https'):
             if proxy := settings.HTTP_PROXIES.get(self.url_scheme):