Add null check on BackofficeUser access

This tag helper "sometimes" throws a null reference exception when accessing the BackofficeUser.
The addition of the null check helps prevent this issue from occurring.

Author: NikRimington

Our.Umbraco.TagHelpers/EditLinkTagHelper.cs

@@ -52,7 +52,7 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
 
             // Check if the user is logged in to the backoffice
             // and they have access to the content section
-            if (_backofficeUserAccessor.BackofficeUser.IsAllowedToSeeEditLink())
+            if (_backofficeUserAccessor?.BackofficeUser != null && _backofficeUserAccessor.BackofficeUser.IsAllowedToSeeEditLink())
             {
                 // Try & get Umbraco Current Node int ID (Only do this if ContentId has NOT been set)
                 if (_umbracoContextAccessor.TryGetUmbracoContext(out var umbracoContext) && ContentId == int.MinValue)
@@ -150,4 +150,4 @@ private static string GetOuterElementStyles(
             return outerStyles.ToString();
         }
     }
-}
+}