Don't crash when webdriverio.getAttribute returns an array of null

Fixes webdriverio-boneyard#43.

index.js

@@ -277,7 +277,9 @@ let isElementsResult = function (result) {
 }
 
 let is$$Result = function (result) {
-    return Array.isArray(result) && result.length && result[0].ELEMENT !== undefined
+    // Commands like getAttribute can return an array of null when the attribute
+    // is not set.
+    return Array.isArray(result) && result.length && result[0] && result[0].ELEMENT !== undefined
 }
 
 /**

test/wrapCommand.spec.js

@@ -18,6 +18,9 @@ WebdriverIO.prototype = {
     getNull: (ms = 50) => new Promise((r) => {
         setTimeout(() => r(null), ms)
     }),
+    getNulls: (ms = 50) => new Promise((r) => {
+        setTimeout(() => r([null, null]), ms)
+    }),
     waitUntilSync: (fn) => new Promise((r) => {
         return wdioSync(fn, r)()
     })
@@ -84,6 +87,12 @@ describe('wrapCommand', () => {
         })
     })
 
+    it('should not treat an array of null as a $$ result', () => {
+        return run(() => {
+            instance.getNulls().should.be.deepEqual([null, null])
+        })
+    })
+
     after(() => {
         delete global.browser
     })