Setting config.customConfig = '' doesn't work

[alfonsoml]

According to the ​docs of customConfig that's the way to avoid an extra request for the config file if it isn't needed, but the code in core/editor.js uses it this way:

	var loadConfig = function( editor )
	{
		var customConfig = CKEDITOR.getUrl( editor.config.customConfig );

		// Check if there is a custom config to load.
		if ( !customConfig )
			return false;

and the call to CKEDITOR.getUrl() returns the base url of CKEditor, so the return false isn't executed.

For this specific problem the "if ( !customConfig )" can be moved before the getUrl call, but maybe it would be better to change that function so it doesn't alter empty strings.

[fredck]

Regression introduced with [4585].

[fredck]

Fixed with [4871].