Execute login page checks that can run without admin dir

Some of the admin checks performed on login page can (and should) be
executed if the admin dir does not exist (e.g. default administrator
account password, detailed error settings).

Fixes #23181

Backported from master-2.5 branch 4980b3b

login_page.php

@@ -146,7 +146,8 @@
 
 $t_warnings = array();
 $t_upgrade_required = false;
-if( config_get_global( 'admin_checks' ) == ON && file_exists( dirname( __FILE__ ) .'/admin' ) ) {
+
+if( config_get_global( 'admin_checks' ) == ON ) {
 	# Check if the admin directory is accessible
 	$t_admin_dir = dirname( __FILE__ ) . '/admin';
 	$t_admin_dir_is_accessible = @file_exists( $t_admin_dir . '/.' );
@@ -200,17 +201,21 @@ function debug_setting_message ( $p_type, $p_setting, $p_value ) {
 	}
 
 	# Check for db upgrade for versions > 1.0.0 using new installer and schema
-	require_once( 'admin' . DIRECTORY_SEPARATOR . 'schema.php' );
-	$t_upgrades_reqd = count( $g_upgrade ) - 1;
+	if( $t_admin_dir_is_accessible ) {
+		require_once( 'admin/schema.php' );
+		$t_upgrades_reqd = count( $g_upgrade ) - 1;
 
-	if( ( 0 < $t_db_version ) &&
+		if( ( 0 < $t_db_version ) &&
 			( $t_db_version != $t_upgrades_reqd ) ) {
 
-		if( $t_db_version < $t_upgrades_reqd ) {
-			$t_warnings[] = lang_get( 'error_database_version_out_of_date_2' );
-			$t_upgrade_required = true;
-		} else {
-			$t_warnings[] = lang_get( 'error_code_version_out_of_date' );
+			if( $t_db_version < $t_upgrades_reqd ) {
+				$t_warnings[] = lang_get( 'error_database_version_out_of_date_2'
+				);
+				$t_upgrade_required = true;
+			}
+			else {
+				$t_warnings[] = lang_get( 'error_code_version_out_of_date' );
+			}
 		}
 	}
 }