spec: update 'BrowserWindow.fromBrowserView(browserView) returns undefined if not attached - returns undefined if not attached' to match docs (null not undefined)

spec/api-browser-window-spec.js

@@ -1014,7 +1014,7 @@ describe('BrowserWindow module', () => {
 
     it('returns undefined if not attached', () => {
       w.setBrowserView(null)
-      assert.strictEqual(BrowserWindow.fromBrowserView(bv), undefined)
+      assert.strictEqual(BrowserWindow.fromBrowserView(bv), null)
     })
   })
 

spec/assert-helpers.js

@@ -3,5 +3,5 @@ function resolverGetters (...args) {
 }
 
 module.exports = {
-  resolverGetters,
+  resolverGetters
 }