interfaces: get_real_interface() as opportunistic try
interfaces: get_real_interface() as opportunistic try

Speeds up lookup.  The caller is ultimately in charge of operating
on the data given by the function.  Most callers do not check the
return value for empty() so let's just fix callers that do require
this.

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core