protected GraphTraversal searchVerticesLabelConstraint(final GraphTraversal t, final String label) {
        return label != null ? t.hasLabel(label) : t;
    }
Translate the Map into a g.V() traversal against the supplied graph. Graph providers will presumably optimize
this traversal to use whatever indices are present and appropriate for efficiency.

Callers are responsible for closing this iterator when finished.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MergeElementStep.java