Minor fixes to Node3DEditorPlugin
Minor fixes to Node3DEditorPlugin

* Change EditorNode3D::update_all_gizmos() to start at the edited scene
  root instead of the whole editor root.
* Call update_all_gizmos() only once at start instead of evrey time a
  gizmo plugin is added.
* Add missing null check.

Faless/godot,Valentactive/godot,guilhermefelipecgs/godot,Faless/godot,ZuBsPaCe/godot,Shockblast/godot,vnen/godot,BastiaanOlij/godot,sanikoyes/godot,Zylann/godot,vnen/godot,guilhermefelipecgs/godot,josempans/godot,sanikoyes/godot,vnen/godot,ZuBsPaCe/godot,firefly2442/godot,Shockblast/godot,josempans/godot,Zylann/godot,josempans/godot,firefly2442/godot,vkbsb/godot,guilhermefelipecgs/godot,DmitriySalnikov/godot,BastiaanOlij/godot,firefly2442/godot,Faless/godot,pkowal1982/godot,pkowal1982/godot,vnen/godot,Shockblast/godot,Valentactive/godot,Shockblast/godot,ZuBsPaCe/godot,akien-mga/godot,vnen/godot,DmitriySalnikov/godot,vnen/godot,ZuBsPaCe/godot,akien-mga/godot,guilhermefelipecgs/godot,pkowal1982/godot,akien-mga/godot,Valentactive/godot,sanikoyes/godot,josempans/godot,akien-mga/godot,Faless/godot,Zylann/godot,akien-mga/godot,vkbsb/godot,godotengine/godot,guilhermefelipecgs/godot,firefly2442/godot,akien-mga/godot,Zylann/godot,BastiaanOlij/godot,pkowal1982/godot,akien-mga/godot,Shockblast/godot,pkowal1982/godot,Zylann/godot,firefly2442/godot,vnen/godot,Valentactive/godot,BastiaanOlij/godot,pkowal1982/godot,Faless/godot,ZuBsPaCe/godot,godotengine/godot,Valentactive/godot,Zylann/godot,sanikoyes/godot,godotengine/godot,Zylann/godot,Valentactive/godot,Faless/godot,Faless/godot,Shockblast/godot,godotengine/godot,guilhermefelipecgs/godot,DmitriySalnikov/godot,akien-mga/godot,BastiaanOlij/godot,sanikoyes/godot,vnen/godot,josempans/godot,vkbsb/godot,Shockblast/godot,BastiaanOlij/godot,DmitriySalnikov/godot,DmitriySalnikov/godot,firefly2442/godot,ZuBsPaCe/godot,vkbsb/godot,godotengine/godot,Valentactive/godot,BastiaanOlij/godot,sanikoyes/godot,pkowal1982/godot,Shockblast/godot,sanikoyes/godot,josempans/godot,vkbsb/godot,guilhermefelipecgs/godot,DmitriySalnikov/godot,ZuBsPaCe/godot,Faless/godot,Zylann/godot,josempans/godot,sanikoyes/godot,godotengine/godot,godotengine/godot,vkbsb/godot,guilhermefelipecgs/godot,vkbsb/godot,godotengine/godot,ZuBsPaCe/godot,firefly2442/godot,BastiaanOlij/godot,firefly2442/godot,vkbsb/godot,pkowal1982/godot,josempans/godot,DmitriySalnikov/godot,Valentactive/godot