SPARQL fails if we encounter a node which is never a subject

[patrickmcelwee]

If one of the connected nodes is only an object (and literally has no triples in which it is a subject), our back-end library errors out because it gets no results for a crucial SPARQL query. I think these lines are unnecessary: https://github.com/patrickmcelwee/mlpm-visjs-graph/blob/master/visjs-lib.xqy#L59-L61 ... we don't have any reason to check to be sure that it is a subject somewhere.

[patrickmcelwee]

This also seems to be connected to the case where there are multiple types for a subject, which causes an exception like this: patrickmcelwee/ml-visjs-graph-ng#19 ... for now, it is probably best to add LIMIT 1 to this line: https://github.com/patrickmcelwee/mlpm-visjs-graph/blob/master/visjs-lib.xqy#L68 ... this will just take a type at random, which perhaps isn't the best, but it is better than breaking.