Ticket #28637: 28637.1.diff

src/wp-includes/theme.php

@@ -955 +955 @@
          */
         $mods[ $name ] = apply_filters( "pre_set_theme_mod_$name", $value, $old_value );
 
+        // If the new value is empty, remove it entirely so that get_theme_mod() returns false
+        if ( '' === $value ) unset( $mods[ $name ] );
+
         $theme = get_option( 'stylesheet' );
         update_option( "theme_mods_$theme", $mods );
 }