Custom Vertex Id (string) on non-existent vertex returns a new vertex

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Potential bug report when using custom vertex id's
Cannot check vertex existence via usual query because a non-existent vertex returns a new vertex instead of null as expected.

- Version: v1.0 branch
- Storage Backend: CQL
- Mixed Index Backend: elasticsearch
- Expected Behavior: gremlin query on non-existent vertex returns null
- Current Behavior: gremlin returns a new empty vertex
- Steps to Reproduce:
a. create a graph with custom vertex id (as string)
1) graph options "set-vertex-id" and "allow-custom-vid-types" = true
2) query for any non-existent vertex: g.V("mycustomid")
3) gremlin returns a vertex with that id and "label=vertex"
4) should return null to indicate vertex not found

Start by reproducing g.V("mycustomid") with the set-vertex-id and allow-custom-vid-types options enabled, using the CQL storage backend and Elasticsearch mixed index. Trace how a custom string ID is resolved when the vertex does not exist. Done means the query returns null rather than a new empty vertex labeled "vertex".

Written by the indexing model from the issue text.