Fix NRE when flushing SaveRAM in GambatteLink for some roms

fixes d788e60
not clear on the reproduction steps, but it will of course involve a rom
which triggers the "no SaveRAM, return null" in Gambatte

src/BizHawk.Emulation.Common/Base Implementations/LinkedSaveRam.cs

@@ -38,7 +38,7 @@ public byte[] CloneSaveRam()
 			int len = 0;
 			for (int i = 0; i < _numCores; i++)
 			{
-				linkedBuffers.Add(_linkedCores[i].AsSaveRam().CloneSaveRam()!);
+				linkedBuffers.Add(_linkedCores[i].AsSaveRam().CloneSaveRam() ?? Array.Empty<byte>());
 				len += linkedBuffers[i].Length;
 			}
 			byte[] ret = new byte[len];
@@ -56,7 +56,9 @@ public void StoreSaveRam(byte[] data)
 			int pos = 0;
 			for (int i = 0; i < _numCores; i++)
 			{
-				var b = new byte[_linkedCores[i].AsSaveRam().CloneSaveRam()!.Length];
+				var toCopy = _linkedCores[i].AsSaveRam().CloneSaveRam(); // wait CloneSaveRam is already a copy, why are we copying it again
+				if (toCopy is null) continue;
+				var b = new byte[toCopy.Length];
 				Buffer.BlockCopy(data, pos, b, 0, b.Length);
 				pos += b.Length;
 				_linkedCores[i].AsSaveRam().StoreSaveRam(b);