revXMLPutIntoNode crashed with empty replacement string

This is the fix for bug 11766, reported by Malte: revXMLPutIntoNode crashes the engine if you try to replace the contents of an existing node with an empty string. This should really be fixed in the thirdparty libxml library, and if I can figure out where to implement a fix I'll submit a pull request to the manager of that library, but this patch at least avoids the crash by replacing the existing string with a carriage return, thus providing a similar effect to eliminating the contents entirely.