Allow the user field to be empty for a login

Not every login is related to a user, e.g. a printer account or similar.

src/users/models/login.py

@@ -12,7 +12,9 @@ class Login(models.Model):
     email = models.CharField(max_length=50, blank=True)
     enabled = models.BooleanField()
     description = models.TextField(blank=True)
-    user = models.ForeignKey(User, on_delete=models.SET_NULL)
+    user = models.ForeignKey(
+        User, on_delete=models.SET_NULL, blank=True, null=True
+    )
     service = models.ForeignKey(
         Service, on_delete=models.SET_NULL, blank=True, null=True
     )