Fix yoda expressions (#574)

+semver:patch

Author: deepsource-autofix[bot]

src/FluentNHibernate/MappingModel/AttributeStore.cs

@@ -60,7 +60,7 @@ public bool Equals(AttributeStore other)
         public override bool Equals(object obj)
         {
            var typed = obj as AttributeStore;
-           if (null == typed) return false;
+           if (typed is null) return false;
            return Equals(typed);
         }
 
@@ -85,4 +85,4 @@ public static T GetOrDefault<T>(this AttributeStore store, string attribute)
             return (T)(store.Get(attribute) ?? default(T));
         }
     }
-}
+}