[reve] Add chek for empty REveGeoPolyShape and REvePolygonSetProjected

[alja]

In some cases of REveGeoShapeExtract, an empty REveGeoShape holder is created for the directory structure.

Example:

   auto all = new  ("CMS");
   all->SetRnrSelf(false);
   all->AddElement(tracker);
   all->AddElement(muon);
   all->AddElement(ecal);
   all->AddElement(hcal);

For this case, EveGlu::TriangleCollector::ProcessData() needs the check before accessing the first element.

[alja]

Below is the stack when importing the shape extract with an empty shape:

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
#1  0x00007ffff7281f63 in __pthread_kill_internal (threadid=<optimized out>, signo=6) at pthread_kill.c:89
#2  0x00007ffff7227f3e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#3  0x00007ffff720f6d0 in __GI_abort () at abort.c:77
#4  0x00007ffff740b084 in std::__glibcxx_assert_fail (file=<optimized out>, line=<optimized out>, function=<optimized out>, condition=<optimized out>) at ../../../../../libstdc++-v3/src/c++11/assert_fail.cc:41
#5  0x00007ffff7852c1e in std::vector<double, std::allocator<double> >::operator[] (this=0x7fffcfe8e690, __n=0) at /usr/include/c++/15/bits/stl_vector.h:1282
#6  0x00007fffd72cb1a9 in ROOT::Experimental::EveGlu::TriangleCollector::ProcessData (this=0x7fffcfe8e600, verts=std::vector of length 0, capacity 0, polys=std::vector of length 0, capacity 0, n_polys=0)
    at /home/alja/root-dev/master/root/graf3d/eve7/src/REveGluTess.cxx:181
#7  0x00007fffd730b77c in ROOT::Experimental::REvePolygonSetProjected::BuildRenderData (this=0x9ca78e0) at /home/alja/root-dev/master/root/graf3d/eve7/src/REvePolygonSetProjected.cxx:121
#8  0x00007fffd72ace22 in ROOT::Experimental::REveElement::WriteCoreJson (this=0x9ca78e0, j=..., rnr_offset=1552) at /home/alja/root-dev/master/root/graf3d/eve7/src/REveElement.cxx:1527
#9  0x00007fffd730b2e0 in ROOT::Experimental::REvePolygonSetProjected::WriteCoreJson (this=0x9ca78e0, j=..., rnr_offset=1552) at /home/alja/root-dev/master/root/graf3d/eve7/src/REvePolygonSetProjected.cxx:73
#10 0x00007fffd7320629 in ROOT::Experimental::REveScene::StreamJsonRecurse (this=0x9c87160, el=0x9ca78e0, jarr=...) at /home/alja/root-dev/master/root/graf3d/eve7/src/REveScene.cxx:189
#11 0x00007fffd7320747 in ROOT::Experimental::REveScene::StreamJsonRecurse (this=0x9c87160, el=0x8ea3650, jarr=...) at /home/alja/root-dev/master/root/graf3d/eve7/src/REveScene.cxx:225
#12 0x00007fffd7320747 in ROOT::Experimental::REveScene::StreamJsonRecurse (this=0x9c87160, el=0x9c87160, jarr=...) at /home/alja/root-dev/master/root/graf3d/eve7/src/REveScene.cxx:225
#13 0x00007fffd7320292 in ROOT::Experimental::REveScene::StreamElements (this=0x9c87160) at /home/alja/root-dev/master/root/graf3d/eve7/src/REveScene.cxx:165
#14 0x00007fffd72d9568 in ROOT::Experimental::REveManager::WindowConnect (this=0x622f2c0, connid=1) at /home/alja/root-dev/master/root/graf3d/eve7/src/REveManager.cxx:780

[alja]

@linev Please check.