Allow ElementTabSet::RemoveTab to work on tabsets with no panels. #1293

Job	Run time
macOS (-DSAMPLES_BACKEND=auto)	6m 40s
Windows (-DSAMPLES_BACKEND=auto -DENABLE_PRECOMPILED_HEADERS=OFF)	5m 43s
macOS (-DSAMPLES_BACKEND=GLFW_GL2)	5m 1s
Linux-legacy (clang, clang++, -DENABLE_PRECOMPILED_HEADERS=OFF -DSAMPLES_BACKEND=GLFW_GL2)	1m 52s
Windows (-DSAMPLES_BACKEND=Win32_VK -DRMLUI_VK_DEBUG=ON)	4m 20s
Linux (clang, clang++, -DENABLE_PRECOMPILED_HEADERS=OFF -DSAMPLES_BACKEND=GLFW_GL2)	1m 51s
Linux-legacy (clang, clang++, -DBUILD_TESTING=ON -DSAMPLES_BACKEND=SDL_VK -DCMAKE_BUILD_TYPE=Debug)	1m 21s
Windows (-DSAMPLES_BACKEND=SDL_VK -DBUILD_LUA_BINDINGS_FOR_LUAJIT=ON)	5m 6s
Linux (clang, clang++, -DBUILD_TESTING=ON -DSAMPLES_BACKEND=SDL_VK -DCMAKE_BUILD_TYPE=Debug)	1m 31s
Linux-legacy (-DBUILD_TESTING=ON)	1m 51s
Linux (-DBUILD_TESTING=ON -DENABLE_PRECOMPILED_HEADERS=OFF, true)	2m 5s
Emscripten	2m 15s
Linux-legacy (-DSAMPLES_BACKEND=SDL_VK -DRMLUI_VK_DEBUG=ON -DENABLE_PRECOMPILED_HEADERS=OFF -DCMA...	1m 51s
Linux (-DNO_FONT_INTERFACE_DEFAULT=ON -DENABLE_LOTTIE_PLUGIN=ON -DSAMPLES_BACKEND=X11_GL2)	1m 19s
Linux (-DDISABLE_RTTI_AND_EXCEPTIONS=ON -DSAMPLES_BACKEND=SDL_GL2)	1m 9s
Linux (-DNO_THIRDPARTY_CONTAINERS=ON -DSAMPLES_BACKEND=SFML_GL2)	1m 17s
Linux (-DSAMPLES_BACKEND=SDL_VK -DRMLUI_VK_DEBUG=ON -DENABLE_PRECOMPILED_HEADERS=OFF -DCMAKE_BUIL...	1m 37s
	46m 49s

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