BUG-650: optimize MapAdaptor a tiny bit

In this context we have already checked the map size, so there is no
need to perform the hasNext() check implied by
Iterables.getOnlyElement().

Change-Id: I12d26053a1bf397b3a026e76c3d07861fd29789c
Signed-off-by: Robert Varga <rovarga@cisco.com>