Multi-Objects: MESH_OT_shortest_path_pick

This fully works as "expected", however it seems strange when
there is no selected vertex in the non-active object.

In 2.7x if you join two monkeys, select a vertex in one of them and
try to use this operator in the other it will select the lonely newly
selected vertex.

If you split both monkeys and do this in 2.8 while multi-editing them
you won't get nothing when trying to ctrl+RMB any vertex in the monkey
object that has nothing selected yet.

I propose to have this addressed in an upcoming patch where we always
select a vertex if no vertex was previously selected.

Improvement to MESH_OT_shortest_path_pick behaviour

Select a vertex/edge/face if there was nothing previously selected
(if the click hit something, of course).