TinyMCE: preserve empty image alt attributes.

Props afercia.
Merges [40110] to the 4.7 branch.
Fixes #39912.
Built from https://develop.svn.wordpress.org/branches/4.7@40112


git-svn-id: http://core.svn.wordpress.org/branches/4.7@40049 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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,

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7.3-alpha-40111';
+$wp_version = '4.7.3-alpha-40112';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.