Prevent customizer header image list from listing user images twice when no theme-specified images exist

fixes #27839. props mcsf

git-svn-id: https://develop.svn.wordpress.org/trunk@28152 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/customize-models.js

@@ -1,4 +1,4 @@
-/* globals _wpCustomizeHeader */
+/* globals _wpCustomizeHeader, _ */
 (function( $, wp ) {
 	var api = wp.customize;
 	api.HeaderTool = {};
@@ -142,7 +142,7 @@
 			}
 
 			// Overridable by an extending class
-			if (!this.data) {
+			if (typeof this.data === 'undefined') {
 				this.data = _wpCustomizeHeader.uploads;
 			}