fix: make validation assertion return void

Author: gajus

src/utilities/validateNextState.js

@@ -1,8 +1,6 @@
-export default (nextState, reducerName: string, action: Object): null => {
+export default (nextState, reducerName: string, action: Object): void => {
   // eslint-disable-next-line no-undefined
   if (nextState === undefined) {
     throw new Error('Reducer "' + reducerName + '" returned undefined when handling "' + action.type + '" action. To ignore an action, you must explicitly return the previous state.');
   }
-
-  return null;
 };

tests/utilities/validateNextState.js

@@ -19,12 +19,13 @@ describe('utilities', () => {
       });
     });
     context('state is defined', () => {
-      it('returns null', () => {
+      it('returns undefined', () => {
         const result = validateNextState(Immutable.Map(), 'reducer name', {
           type: 'foo'
         });
 
-        expect(result).to.equal(null);
+        // eslint-disable-next-line no-undefined
+        expect(result).to.equal(undefined);
       });
     });
   });