Only pass arguments from the deprecated add_custom_background() or...

Only pass arguments from the deprecated add_custom_background() or add_custom_image_header() to add_theme_support() if the argument was actually supplied. With [20212], default argument values (like wp-head-callback=_custom_background_cb) only work for arguments that were not explicitly set, so don't set empty values. see #20249.

git-svn-id: https://core.svn.wordpress.org/trunk@20246 1a063a9b-81f0-0310-95a4-ce76da25c4cd