Make default graphics backend not show up as empty

Fixes https://bugs.dolphin-emu.org/issues/12245.

I considered making a change to DolphinQt instead of
the core, but then additional effort would've been
required to add the same fix to the Android GUI once
we start using the new config system there.

Source/Core/Core/Config/MainSettings.cpp

@@ -12,6 +12,7 @@
 #include "Core/HW/Memmap.h"
 #include "Core/HW/SI/SI_Device.h"
 #include "Core/PowerPC/PowerPC.h"
+#include "VideoCommon/VideoBackendBase.h"
 
 namespace Config
 {
@@ -97,7 +98,8 @@ const Info<bool> MAIN_OVERCLOCK_ENABLE{{System::Main, "Core", "OverclockEnable"}
 const Info<bool> MAIN_RAM_OVERRIDE_ENABLE{{System::Main, "Core", "RAMOverrideEnable"}, false};
 const Info<u32> MAIN_MEM1_SIZE{{System::Main, "Core", "MEM1Size"}, Memory::MEM1_SIZE_RETAIL};
 const Info<u32> MAIN_MEM2_SIZE{{System::Main, "Core", "MEM2Size"}, Memory::MEM2_SIZE_RETAIL};
-const Info<std::string> MAIN_GFX_BACKEND{{System::Main, "Core", "GFXBackend"}, ""};
+const Info<std::string> MAIN_GFX_BACKEND{{System::Main, "Core", "GFXBackend"},
+                                         VideoBackendBase::GetDefaultBackendName()};
 const Info<std::string> MAIN_GPU_DETERMINISM_MODE{{System::Main, "Core", "GPUDeterminismMode"},
                                                   "auto"};
 const Info<std::string> MAIN_PERF_MAP_DIR{{System::Main, "Core", "PerfMapDir"}, ""};