Stop crashing on undefined or null properties

Similar to other fix for flow intersection (jsx-eslint#1806)

Author: nicholas-l

lib/rules/prop-types.js

@@ -360,7 +360,7 @@ module.exports = {
         and property value. (key, value) => void
      */
     function iterateProperties(properties, fn) {
-      if (properties.length && typeof fn === 'function') {
+      if (properties && properties.length && typeof fn === 'function') {
         for (let i = 0, j = properties.length; i < j; i++) {
           const node = properties[i];
           const key = getKeyValue(node);