diagrams_ does not accept an empty set of external momenta

[tueda]

The following program does not generate any vacuum bubble diagrams:

Model PHI3;
  Particle phi,1;
  Vertex phi,phi,phi:g;
EndModel;
V p1,p2,p3;
L F = diagrams_(PHI3,{},{},{},{p1,p2,p3},2);
P +s;
.end

   F =
       + diagrams_(PHI3,,,,,2)
      ;

Note that it works if I supply an unused dummy vector:

Model PHI3;
  Particle phi,1;
  Vertex phi,phi,phi:g;
EndModel;
V p1,p2,p3;
L F = diagrams_(PHI3,{},{},{parg_},{p1,p2,p3},2);
P +s;
.end

   F =
       + 1/8*topo_(1)*node_(1,g,phi(-p1),phi(p1),phi(-p2))*node_(2,g,phi(p2),
      phi(-p3),phi(p3))
       + 1/12*topo_(2)*node_(1,g,phi(-p1),phi(-p2),phi(-p3))*node_(2,g,phi(p1)
      ,phi(p2),phi(p3))
      ;

[jodavies]

This looks straightforward to resolve, I will include it in #761 . Related, is that if either external or internal sets don't contain sufficient momenta, is that FORM crashes rather than give a useful error. I'll try to resolve that as well.