fix(createContextHandler): handler precondition check allows passing …

…null/undefined which works w be

affects: @tao.js/react

createContextHandler allows for a context handler not to have a handler function
w behavior that it'll manage setting the state
-> this behavior could not previously be used b/c of the check

packages/react-tao/src/createContextHandler.js

@@ -6,7 +6,7 @@ import { normalizeAC, cleanInput } from './helpers';
 import { Context } from './Provider';
 
 export default function createContextHandler(tao, handler, defaultValue) {
-  if (typeof handler !== 'function') {
+  if (handler != null && typeof handler !== 'function') {
     throw new Error('createContextHandler `handler` must be a function');
   }
   const WrappingContext = React.createContext(defaultValue);