JanusGraphTransaction#getVertex throws exception when vertex does not exist

According to the javadocs this method should return null if the vertex does not exist, instead it throws an exception due to an assertion error in the guava cache:

java.lang.AssertionError: Should not happen: CacheLoader returned null for key 82931045.
at org.janusgraph.graphdb.transaction.vertexcache.GuavaVertexCache.get(GuavaVertexCache.java:84)
at org.janusgraph.graphdb.transaction.StandardJanusGraphTx.getVertex(StandardJanusGraphTx.java:394)

Start with StandardJanusGraphTx.getVertex and follow the call into GuavaVertexCache.get, using the reported stack trace and nonexistent vertex case as the reproduction. Confirm the behavior against the getVertex javadocs; done means a missing vertex returns null instead of raising the Guava cache assertion.

Written by the indexing model from the issue text.