[JENKINS-60750] Skip CONFIGURE_INSTANCE panel when root URL is set during skipFirstUser (#4430)

* [JENKINS-60750] Skip the CONFIGURE_INSTANCE panel when JenkinsLocationConfiguration.url is set, even when skipFirstUser is called.

* Add nullability annotation
Co-Authored-By: NOleg Nenashev <o.v.nenashev@gmail.com>
Co-authored-by: NOleg Nenashev <o.v.nenashev@gmail.com>

core/src/main/java/jenkins/model/Jenkins.java

@@ -304,6 +304,7 @@ import hudson.util.LogTaskListener;
 import static java.util.logging.Level.*;
 import javax.annotation.Nonnegative;
 import static javax.servlet.http.HttpServletResponse.*;
+import org.kohsuke.accmod.restrictions.DoNotUse;
 import org.kohsuke.stapler.WebMethod;
 
 /**
@@ -2340,6 +2341,15 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
         return null;
     }
 
+    /** Exported alias for {@link JenkinsLocationConfiguration#getUrl}. */
+    @Exported(name="url")
+    @Restricted(DoNotUse.class)
+    @CheckForNull
+    public String getConfiguredRootUrl() {
+        JenkinsLocationConfiguration config = JenkinsLocationConfiguration.get();
+        return config != null ? config.getUrl() : null;
+    }
+
     /**
      * Is Jenkins running in HTTPS?
      *

war/src/main/js/pluginSetupWizardGui.js

@@ -940,7 +940,19 @@ var createPluginSetupWizard = function(appendTarget) {
 	var skipFirstUser = function() {
 		$('button').prop({disabled:true});
 		firstUserSkipped = true;
-		showConfigureInstance();
+                jenkins.get('/api/json?tree=url', function(data) {
+                    if (data.url) {
+                        // as in InstallState.ConfigureInstance.initializeState
+                        showSetupCompletePanel({message: translations.installWizard_firstUserSkippedMessage});
+                    } else {
+                        showConfigureInstance();
+                    }
+                }, {
+                    error: function() {
+                        // give up
+                        showConfigureInstance();
+                    }
+                });
 	};
 	
 	var handleConfigureInstanceResponseSuccess = function (data) {