bridge: Don't crash when unable to retrieve PCP instance names
bridge: Don't crash when unable to retrieve PCP instance names

Passing NULL to json_node_init_string results in a crash.

Fixes #2850
Closes #2854
Reviewed-by: Stef Walter <8a7fbf0f15d3ad982af54ef28f5c117d2494b76c@redhat.com>

martinpitt/cockpit,martinpitt/cockpit,xhad/cockpit,larsu/cockpit,garrett/cockpit,martinpitt/cockpit,matobet/cockpit,michalskrivanek/cockpit,petervo/cockpit,matobet/cockpit,haiyangd/cockpit_view,FireDrunk/cockpit,jscotka/cockpit,petervo/cockpit,matobet/cockpit,petervo/cockpit,xhad/cockpit,larsu/cockpit,stefwalter/cockpit,Scribery/cockpit,sgallagher/cockpit,moraleslazaro/cockpit,vanloswang/cockpit,andreasn/cockpit,arilivigni/cockpit,sgallagher/cockpit,cockpit-project/cockpit,stefwalter/cockpit,cockpituous/cockpit,vanloswang/cockpit,cockpituous/cockpit,vanloswang/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,matobet/cockpit,SotolitoLabs/cockpit,SotolitoLabs/cockpit,xhad/cockpit,stefwalter/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,dperpeet/cockpit,harishanand95/cockpit,Scribery/cockpit,Scribery/cockpit,sgallagher/cockpit,petervo/cockpit,moraleslazaro/cockpit,garrett/cockpit,andreasn/cockpit,garrett/cockpit,deryni/cockpit,matobet/cockpit,andreasn/cockpit,larskarlitski/cockpit,vanloswang/cockpit,moraleslazaro/cockpit,harishanand95/cockpit,sgallagher/cockpit,haiyangd/cockpit_view,deryni/cockpit,larsu/cockpit,mvollmer/cockpit,matobet/cockpit,deryni/cockpit,jscotka/cockpit,mvollmer/cockpit,harishanand95/cockpit,dperpeet/cockpit,xhad/cockpit,FireDrunk/cockpit,michalskrivanek/cockpit,Scribery/cockpit,larsu/cockpit,vanloswang/cockpit,jscotka/cockpit,xhad/cockpit,xhad/cockpit,larsu/cockpit,mareklibra/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,larsu/cockpit,arilivigni/cockpit,cockpit-project/cockpit,petervo/cockpit,larskarlitski/cockpit,haiyangd/cockpit_view,sgallagher/cockpit,FireDrunk/cockpit,cockpit-project/cockpit,jscotka/cockpit,sgallagher/cockpit,martinpitt/cockpit,jscotka/cockpit,larskarlitski/cockpit,cockpit-project/cockpit,mareklibra/cockpit,larskarlitski/cockpit,SotolitoLabs/cockpit,deryni/cockpit,michalskrivanek/cockpit,jscotka/cockpit,mvollmer/cockpit,FireDrunk/cockpit,michalskrivanek/cockpit,michalskrivanek/cockpit,harishanand95/cockpit,cockpit-project/cockpit,vanloswang/cockpit,stefwalter/cockpit,xhad/cockpit,dperpeet/cockpit,michalskrivanek/cockpit,stefwalter/cockpit,Scribery/cockpit,mvollmer/cockpit,mareklibra/cockpit,deryni/cockpit,arilivigni/cockpit,moraleslazaro/cockpit,arilivigni/cockpit,michalskrivanek/cockpit,Scribery/cockpit,haiyangd/cockpit_view,dperpeet/cockpit,harishanand95/cockpit,garrett/cockpit,Scribery/cockpit,dperpeet/cockpit,dperpeet/cockpit,larskarlitski/cockpit,petervo/cockpit,stefwalter/cockpit,cockpituous/cockpit,cockpituous/cockpit,vanloswang/cockpit,arilivigni/cockpit,arilivigni/cockpit,andreasn/cockpit,petervo/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,sgallagher/cockpit,larskarlitski/cockpit,cockpituous/cockpit,andreasn/cockpit,andreasn/cockpit,FireDrunk/cockpit,garrett/cockpit,stefwalter/cockpit,harishanand95/cockpit,moraleslazaro/cockpit,haiyangd/cockpit_view,mvollmer/cockpit,martinpitt/cockpit,dperpeet/cockpit,larskarlitski/cockpit,mareklibra/cockpit,harishanand95/cockpit,arilivigni/cockpit,mareklibra/cockpit