Simplify the SRAM's representation in NDSCartArgs (#1914)

* Simplify the SRAM's representation in  `NDSCartArgs`

- I overthought this one.
- I could've just checked `args && args->SRAM`, but then some other poor bastard might make this mistake.
- Don't mix `pair`, `optional`, and `unique_ptr` all at once, kids.

* Fix a `nullptr` read

src/frontend/qt_sdl/ROMManager.cpp

@@ -1316,8 +1316,8 @@ bool LoadROM(EmuThread* emuthread, QStringList filepath, bool reset)
         // the ROM is homebrew or not.
         // So this is the card we *would* load if the ROM were homebrew.
         .SDCard = GetDLDISDCardArgs(),
-
-        .SRAM = std::make_pair(std::move(savedata), savelen),
+        .SRAM = std::move(savedata),
+        .SRAMLength = savelen,
     };
 
     auto cart = NDSCart::ParseROM(std::move(filedata), filelen, std::move(cartargs));