[Feature request] Allow for default class where a rdf:Class does not exist.

[charlesivie]

Is your feature request related to a problem? Please describe. When data does not have a class type e.g. nothing like :Charlie a :Person The user interface cannot display a node, as it is reliant on that data to be in the model.

Describe the solution you'd like Where a class does not exist for a node, assume a default class type of rdf:Resource. Update the SPARQL queries accordingly using 'Coalesce'.

[xiazcy]

Relevant issue: https://github.com/aws/graph-explorer/issues/185.