Do nothing if there are no extra virtual apps.
Do nothing if there are no extra virtual apps.

Before this change even if there are no virtual apps the DAG is reloaded, in our system this takes ~3 seconds. With this change we give up before loading the DAG if there are no virtual apps.  Rebar3 behavior should not change other than better performance. This change has been tested by running rebar3 ct

rebar/rebar3,erlang/rebar3,rebar/rebar3,saleyn/rebar3,lrascao/rebar3