[MergeDiff] Add more catches for empty/null lists

DATabase/MergeDiff.cs

@@ -390,6 +390,8 @@ namespace SabreTools
 					List<RomData> oldroms = DatTools.Merge(userData.Roms[key], _logger);
 					List<RomData> newroms = new List<RomData>();
 
+					if (oldroms != null && oldroms.Count > 0)
+					{
 						foreach (RomData rom in oldroms)
 						{
 							if (rom.SystemID == j)
@@ -410,6 +412,8 @@ namespace SabreTools
 								newroms.Add(rom);
 							}
 						}
+					}
+					
 					userData.Roms[key] = newroms;
 				}