Don't resolve empty src/preload attribute values

lib/renderer/web-view/web-view-attributes.js

@@ -9,6 +9,7 @@ const remote = require('electron').remote
 var a = document.createElement('a')
 
 var resolveURL = function (url) {
+  if (url === '') return ''
   a.href = url
   return a.href
 }

spec/webview-spec.js

@@ -80,6 +80,10 @@ describe('<webview> tag', function () {
       assert.equal(webview.src, '')
       webview.src = ''
       assert.equal(webview.src, '')
+      webview.src = null
+      assert.equal(webview.src, '')
+      webview.src = undefined
+      assert.equal(webview.src, '')
     })
   })
 
@@ -209,6 +213,10 @@ describe('<webview> tag', function () {
       assert.equal(webview.preload, '')
       webview.preload = ''
       assert.equal(webview.preload, '')
+      webview.preload = null
+      assert.equal(webview.preload, '')
+      webview.preload = undefined
+      assert.equal(webview.preload, '')
     })
   })