IO: AddInputCharacters function ignore 0 input. (#3252)
IO: AddInputCharacters function ignore 0 input. (#3252)

Amend ef13d954 + c8ea0a01 (#2541, #2538, #2815)

bkaradzic/imgui,ocornut/imgui,nProtect/imgui,ggtucker/imgui,ggtucker/imgui,bkaradzic/imgui,ggtucker/imgui,bkaradzic/imgui,ocornut/imgui,ocornut/imgui,aimotive/imgui-glfw,Flix01/imgui,aimotive/imgui-glfw,Flix01/imgui,nProtect/imgui,Flix01/imgui,nProtect/imgui