Fix setting up signing prefix

Author: alexeysemenyukoracle

src/jdk.jpackage/macosx/classes/jdk/jpackage/internal/SigningConfigBuilder.java

@@ -57,6 +57,8 @@ SigningConfigBuilder signingIdentityPrefix(LauncherStartupInfo mainLauncherStart
         final var pkgName = mainLauncherStartupInfo.packageName();
         if (!pkgName.isEmpty()) {
             signingIdentityPrefix(pkgName + ".");
+        } else {
+            signingIdentityPrefix(mainLauncherStartupInfo.simpleClassName() + ".");
         }
         return this;
     }