OGM wrapper does not set internal _type property of list relationships if they are null

[oxisto]

The OGM wrapper seems to stored internally the type of a list relationship in the property field_name, to instantiate it later with the correct list/collection-subtype. However, there is an issue, if a collection element, that is considered as a relationship, such as the newly introduced annotations field of a Node is null. More specifically, the function vertexToNode crashes because it will look for the property type here:

codyze/src/main/java/de/fraunhofer/aisec/crymlin/connectors/db/OverflowDatabase.java

Lines 347 to 348 in 0d6b6e7

	className = (String) v.property(f.getName() + "_type").value();
	collectionType = Class.forName(className);

However, the createEdges function only sets this special property, if the value of that property is not null, probably to not save unnecessary properties.

codyze/src/main/java/de/fraunhofer/aisec/crymlin/connectors/db/OverflowDatabase.java

Line 621 in 0d6b6e7

v.property(f.getName() + "_type", x.getClass().getName());

Even this would be caught by an exception, but this exception tries to rely on the fact that the node property is there, which is not mandatory and thus, the caught exception triggers an exception