BUG-650: squash maps to Collections.emptyMap()

If possible, we do not retain mutable maps, but rather short-cut to
emptyMap(). This should help us a bit when traversing empty elements.

Change-Id: Iff7eccfcc99c451d11f87cf466c7f09bde63ee10
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-650: squash maps to Collections.emptyMap()"