[Fix] Don't falsely identify signs as TC signs that dont contain a type

The Issue:
Signs that contain, for example, the word "wait" in the second line were erratically treated as TrainCarts sign and thus destroyed for not having a type. Since this has the potential to break other plugins (and also innocent signs which never aspired to have anything to do with any plugins), this PR changes the detection of the sign type to only yield a result if the Sign has a type different from NONE.