Ignore empty locale list when diffing Configuration objects

b46fdd42 introduced the concept of locale list to Configuration objects.
However, the change alway returned a locale difference when the delta
object locale list is set to the defualt (empty). We now don't return
a difference if the delta object as an empty locale list which is
consistent with what we did before the change and in line with how we
diff other configuration fields.

Bug: 23744091
Change-Id: I8c2c39d9a7e91e6a3a283312b745c4fdf6874396