fix: no knobs after select

* upgraded storybook to support undefined in select

.gitignore

@@ -2,3 +2,4 @@ node_modules/
 dist
 settings.json
 .idea
+storybook-static

package.json

@@ -11,6 +11,7 @@
     "compile": "babel ./src --out-dir ./dist",
     "prepublish": "npm run clean && npm run compile",
     "build-storybook": "build-storybook",
+    "build": "yarn build-storybook",
     "storybook": "start-storybook -p 9010",
     "test": "jest"
   },

src/index.js

@@ -42,7 +42,7 @@ typeKnobsMap.forEach(({ name, knob, args = [] }, weight) => addKnobResolver({
 }))
 
 const optionsReducer = (res, value) => ({ ...res, [value]: value })
-const withDefaultOption = (options) => ({ '--': null, ...options })
+const withDefaultOption = (options) => ({ '--': undefined, ...options })
 const createSelect = (propName, elements, defaultProps, isRequired) => {
   try {
     const options = elements