Fix GLTF crash when the material is not set.
Fix GLTF crash when the material is not set.

Sometimes there are meshes that doesn't have materials, so make sure to check this case before extracting the name.
Zylann/godot,guilhermefelipecgs/godot,pkowal1982/godot,sanikoyes/godot,DmitriySalnikov/godot,josempans/godot,Shockblast/godot,sanikoyes/godot,pkowal1982/godot,BastiaanOlij/godot,Faless/godot,ZuBsPaCe/godot,honix/godot,Shockblast/godot,ZuBsPaCe/godot,pkowal1982/godot,Valentactive/godot,BastiaanOlij/godot,akien-mga/godot,pkowal1982/godot,guilhermefelipecgs/godot,guilhermefelipecgs/godot,firefly2442/godot,godotengine/godot,Shockblast/godot,honix/godot,BastiaanOlij/godot,akien-mga/godot,josempans/godot,vnen/godot,sanikoyes/godot,BastiaanOlij/godot,Faless/godot,godotengine/godot,Zylann/godot,pkowal1982/godot,Valentactive/godot,josempans/godot,vkbsb/godot,firefly2442/godot,Valentactive/godot,firefly2442/godot,Valentactive/godot,Faless/godot,DmitriySalnikov/godot,josempans/godot,Faless/godot,ZuBsPaCe/godot,honix/godot,sanikoyes/godot,honix/godot,firefly2442/godot,vkbsb/godot,vnen/godot,akien-mga/godot,Valentactive/godot,firefly2442/godot,Zylann/godot,sanikoyes/godot,BastiaanOlij/godot,akien-mga/godot,BastiaanOlij/godot,Faless/godot,ZuBsPaCe/godot,godotengine/godot,vnen/godot,guilhermefelipecgs/godot,vnen/godot,akien-mga/godot,godotengine/godot,vnen/godot,Valentactive/godot,firefly2442/godot,akien-mga/godot,guilhermefelipecgs/godot,vnen/godot,vkbsb/godot,sanikoyes/godot,BastiaanOlij/godot,josempans/godot,DmitriySalnikov/godot,akien-mga/godot,josempans/godot,Zylann/godot,guilhermefelipecgs/godot,ZuBsPaCe/godot,godotengine/godot,firefly2442/godot,sanikoyes/godot,godotengine/godot,vkbsb/godot,josempans/godot,Valentactive/godot,vnen/godot,Zylann/godot,josempans/godot,DmitriySalnikov/godot,DmitriySalnikov/godot,Zylann/godot,ZuBsPaCe/godot,vkbsb/godot,guilhermefelipecgs/godot,Shockblast/godot,pkowal1982/godot,Valentactive/godot,ZuBsPaCe/godot,Shockblast/godot,Shockblast/godot,Zylann/godot,DmitriySalnikov/godot,BastiaanOlij/godot,pkowal1982/godot,Faless/godot,DmitriySalnikov/godot,Faless/godot,vkbsb/godot,akien-mga/godot,pkowal1982/godot,Zylann/godot,honix/godot,Shockblast/godot,godotengine/godot,sanikoyes/godot,vkbsb/godot,honix/godot,vkbsb/godot,Faless/godot,vnen/godot,godotengine/godot,Shockblast/godot,guilhermefelipecgs/godot,ZuBsPaCe/godot,firefly2442/godot