Guard against setting empty XCODE_DEVELOPER_DIR_PATH env var

Only set the XCODE_DEVELOPER_DIR_PATH environment variable when
DeveloperRoot is non-empty. Setting it to an empty string could
confuse child processes that check for its presence.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rmarinho

Xamarin.MacDev/AppleSdkSettings.cs

@@ -194,7 +194,8 @@ public static void Init ()
 				XcodePath = xcode;
 				DeveloperRoot = devroot;
 				DeveloperRootVersionPlist = vplist;
-				Environment.SetEnvironmentVariable ("XCODE_DEVELOPER_DIR_PATH", DeveloperRoot);
+				if (!string.IsNullOrEmpty (DeveloperRoot))
+					Environment.SetEnvironmentVariable ("XCODE_DEVELOPER_DIR_PATH", DeveloperRoot);
 			} else {
 				SetInvalid ();
 				return;