make selecting user shaders not crash with the default empty string path

Path.GetDirectoryName throws if it is handed an empty string apparently

src/BizHawk.Client.EmuHawk/config/DisplayConfig.cs

@@ -270,7 +270,8 @@ namespace BizHawk.Client.EmuHawk
 		{
 			var result = this.ShowFileOpenDialog(
 				filter: CgShaderPresetsFSFilterSet,
-				initDir: Path.GetDirectoryName(_pathSelection)!,
+				initDir: string.IsNullOrWhiteSpace(_pathSelection)
+				? string.Empty : Path.GetDirectoryName(_pathSelection)!,
 				initFileName: _pathSelection);
 			if (result is null) return;