Fix conversion error with new ImGui version

imgui-SFML.cpp

@@ -810,7 +810,7 @@ ImVec2 getDownRightAbsolute(const sf::FloatRect& rect) {
 }
 
 ImTextureID convertGLTextureHandleToImTextureID(GLuint glTextureHandle) {
-    ImTextureID textureID = nullptr;
+    ImTextureID textureID{};
     std::memcpy(&textureID, &glTextureHandle, sizeof(GLuint));
     return textureID;
 }