TinyMCE: preserve empty image alt attributes.

Props afercia.
Merges [40110] to the 4.7 branch.
Fixes #39912.

git-svn-id: https://develop.svn.wordpress.org/branches/4.7@40112 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/tinymce/plugins/wpeditimage/plugin.js

@@ -382,13 +382,15 @@ tinymce.PluginManager.add( 'wpeditimage', function( editor ) {
 			src: imageData.url,
 			width: width || null,
 			height: height || null,
-			alt: imageData.alt,
 			title: imageData.title || null,
 			'class': classes.join( ' ' ) || null
 		};
 
 		dom.setAttribs( imageNode, attrs );
 
+		// Preserve empty alt attributes.
+		editor.$( imageNode ).attr( 'alt', imageData.alt || '' );
+
 		linkAttrs = {
 			href: imageData.linkUrl,
 			rel: imageData.linkRel || null,