Empty turtle file after export

[SvenLieber]

How to reproduce the behavior:

Open the app and press the +Shape button to create a node shape and then press the +Property button to create a property shape.
Then go to the menu and export the SHACL file as turtle.
The turtle file is empty.

It works for json.

[fbrckman]

As stated in #145:

In N3Serializer.serialize, jsonld.toRDF(model) returns an empty array in some cases. The model probably does not meet certain requirements. This is because ID's starting with a digit are treated as relative IRI's. Generated uuid's have a chance to start with a digit, causing them to be ignored since they are "invalid".

As of #198 (commit 944e221), ID's have to be valid IRI's to be accepted. This means that ID's cannot start with a digit anymore so that an empty Turtle file is no longer an issue.