Fix -Wconversion warning (#2379)
Fix -Wconversion warning (#2379)

The warning was caused by implicit conversion from pointer type which
NULL has to non-pointer type, e.g. if ImTextureID is long int
aimotive/imgui-glfw,ggtucker/imgui,ggtucker/imgui,bkaradzic/imgui,bkaradzic/imgui,cmaughan/imgui,nProtect/imgui,aimotive/imgui-glfw,bkaradzic/imgui,ocornut/imgui,nProtect/imgui,ocornut/imgui,Flix01/imgui,nProtect/imgui,cmaughan/imgui,ocornut/imgui,Flix01/imgui,dougbinks/imgui,dougbinks/imgui,cmaughan/imgui,Flix01/imgui,burningreggae/imgui,ggtucker/imgui,burningreggae/imgui,cmaughan/imgui