Fix crash when converting empty scene to TileSet
Fix crash when converting empty scene to TileSet

Closes #12708.
Cherry-picked from #12732.

godotengine/godot,firefly2442/godot,BastiaanOlij/godot,BastiaanOlij/godot,josempans/godot,ZuBsPaCe/godot,firefly2442/godot,okamstudio/godot,sanikoyes/godot,BastiaanOlij/godot,mcanders/godot,vkbsb/godot,mcanders/godot,firefly2442/godot,NateWardawg/godot,ex/godot,sanikoyes/godot,guilhermefelipecgs/godot,okamstudio/godot,honix/godot,godotengine/godot,DmitriySalnikov/godot,groud/godot,Paulloz/godot,firefly2442/godot,pkowal1982/godot,honix/godot,groud/godot,n-pigeon/godot,MarianoGnu/godot,Zylann/godot,ZuBsPaCe/godot,ZuBsPaCe/godot,ex/godot,RandomShaper/godot,RandomShaper/godot,Faless/godot,Zylann/godot,NateWardawg/godot,Valentactive/godot,sanikoyes/godot,ex/godot,pkowal1982/godot,Shockblast/godot,godotengine/godot,akien-mga/godot,NateWardawg/godot,guilhermefelipecgs/godot,groud/godot,godotengine/godot,akien-mga/godot,n-pigeon/godot,pkowal1982/godot,Paulloz/godot,josempans/godot,Shockblast/godot,akien-mga/godot,BastiaanOlij/godot,NateWardawg/godot,ZuBsPaCe/godot,NateWardawg/godot,josempans/godot,Faless/godot,ZuBsPaCe/godot,groud/godot,godotengine/godot,pkowal1982/godot,pkowal1982/godot,pkowal1982/godot,BastiaanOlij/godot,Faless/godot,pkowal1982/godot,RandomShaper/godot,sanikoyes/godot,Valentactive/godot,vkbsb/godot,vkbsb/godot,Zylann/godot,guilhermefelipecgs/godot,firefly2442/godot,Zylann/godot,MarianoGnu/godot,MarianoGnu/godot,firefly2442/godot,ZuBsPaCe/godot,guilhermefelipecgs/godot,pkowal1982/godot,Valentactive/godot,akien-mga/godot,Shockblast/godot,Paulloz/godot,vnen/godot,okamstudio/godot,okamstudio/godot,MarianoGnu/godot,groud/godot,mcanders/godot,DmitriySalnikov/godot,NateWardawg/godot,NateWardawg/godot,ZuBsPaCe/godot,ex/godot,vkbsb/godot,sanikoyes/godot,josempans/godot,MarianoGnu/godot,Zylann/godot,josempans/godot,akien-mga/godot,mcanders/godot,n-pigeon/godot,okamstudio/godot,honix/godot,josempans/godot,ZuBsPaCe/godot,Shockblast/godot,BastiaanOlij/godot,Zylann/godot,MarianoGnu/godot,n-pigeon/godot,Valentactive/godot,Valentactive/godot,DmitriySalnikov/godot,vkbsb/godot,MarianoGnu/godot,okamstudio/godot,RandomShaper/godot,groud/godot,firefly2442/godot,josempans/godot,MarianoGnu/godot,vnen/godot,Paulloz/godot,ex/godot,vnen/godot,Shockblast/godot,firefly2442/godot,sanikoyes/godot,okamstudio/godot,godotengine/godot,DmitriySalnikov/godot,Paulloz/godot,mcanders/godot,vkbsb/godot,Faless/godot,Zylann/godot,okamstudio/godot,RandomShaper/godot,vkbsb/godot,Shockblast/godot,honix/godot,okamstudio/godot,BastiaanOlij/godot,DmitriySalnikov/godot,guilhermefelipecgs/godot,Faless/godot,guilhermefelipecgs/godot,ex/godot,Valentactive/godot,DmitriySalnikov/godot,okamstudio/godot,RandomShaper/godot,ex/godot,BastiaanOlij/godot,Shockblast/godot,RandomShaper/godot,akien-mga/godot,Shockblast/godot,josempans/godot,vkbsb/godot,mcanders/godot,sanikoyes/godot,vnen/godot,guilhermefelipecgs/godot,Paulloz/godot,honix/godot,n-pigeon/godot,vnen/godot,DmitriySalnikov/godot,ex/godot,godotengine/godot,vnen/godot,akien-mga/godot,Zylann/godot,Faless/godot,n-pigeon/godot,n-pigeon/godot,Valentactive/godot,akien-mga/godot,NateWardawg/godot,Faless/godot,honix/godot,guilhermefelipecgs/godot,vnen/godot,sanikoyes/godot,Valentactive/godot,godotengine/godot,Paulloz/godot,vnen/godot,NateWardawg/godot,Faless/godot