Fix glTF scene export crash on null normal texture
Fix glTF scene export crash on null normal texture

Also removes a redundant get_texture call directly below
the modified code block.

Fixes #56379

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