Handle particle systems that don't have emitters

Fixes a crash in the Magic Diversity mod.

components/nifosg/nifloader.cpp

@@ -825,6 +825,8 @@ namespace NifOsg
 
             partsys->setFreezeOnCull(true);
 
+            if (!partctrl->emitter.empty())
+            {
                 osg::ref_ptr<Emitter> emitter = handleParticleEmitter(partctrl);
                 emitter->setParticleSystem(partsys);
                 emitter->setReferenceFrame(osgParticle::ParticleProcessor::RELATIVE_RF);
@@ -849,6 +851,7 @@ namespace NifOsg
                 osg::ref_ptr<ParticleSystemController> callback(new ParticleSystemController(partctrl));
                 setupController(partctrl, callback, animflags);
                 emitter->setUpdateCallback(callback);
+            }
 
             // affectors must be attached *after* the emitter in the scene graph for correct update order
             // attach to same node as the ParticleSystem, we need osgParticle Operators to get the correct