Crash on empty UI.#114

## Description When you have an empty UI scene I get the following crash: ``` unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX` stack backtrace: 0: rust_begin_unwind at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/std/src/panicking.rs:662:5 1: core::panicking::panic_nounwind_fmt::runtime at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/panicking.rs:112:18 2: core::panicking::panic_nounwind_fmt at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/panicking.rs:122:5 3: core::panicking::panic_nounwind at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/panicking.rs:221:5 4: core::slice::raw::from_raw_parts::precondition_check at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/ub_checks.rs:70:21 5: core::slice::raw::from_raw_parts at /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/ub_checks.rs:77:17 6: imgui::render::draw_data::DrawData::cmd_lists at /home/toaster/.cargo/registry/src/index.crates.io-6f17d22bba15001f/imgui-0.12.0/src/render/draw_data.rs:59:9 7: imgui::render::draw_data::DrawData::draw_lists at /home/toaster/.cargo/registry/src/index.crates.io-6f17d22bba15001f/imgui-0.12.0/src/render/draw_data.rs:48:23 8: imgui_wgpu::Renderer::prepare at /home/toaster/.cargo/git/checkouts/imgui-wgpu-rs-7931b819800fb253/bab0725/src/lib.rs:585:26 9: imgui_wgpu::Renderer::render at /home/toaster/.cargo/git/checkouts/imgui-wgpu-rs-7931b819800fb253/bab0725/src/lib.rs:693:33 10: eos_engine::ui::Ui::render at ./src/ui/mod.rs:141:9 11: eos_engine::render::pipeline_manager::PipelineManager::render at ./src/render/pipeline_manager.rs:174:13 12: eos_engine::render::RenderState::render at ./src/render/mod.rs:235:17 13: eos_engine::app::App::run::{{closure}}::{{closure}} at ./src/app.rs:417:29 ``` You can bypass this by adding your own check to see if you've written UI, however this causes another another issue: ``` game: ./third-party/imgui-master/imgui/imgui.cpp:8880: void ImGui::ErrorCheckNewFrameSanityChecks(): Assertion `(g.FrameCount == 0 || g.FrameCountEnded == g.FrameCount) && "Forgot to call Render() or EndFrame() at the end of the previous frame?"' failed. ``` ## Repro steps Run without calling any ui functions. ## Extra Materials No ## System Information Linux/x11/wayland/nvidia