[5.0] Show proper error message when package is installed with no ext…

…ensions (#42337)

* Update PackageAdapter.php

Avoid fatal PHP error on malformed manifest.

* Update PackageAdapter.php

Fixed issue which caused JSON parse error.

* Update PackageAdapter.php

Removed duplicated code.

libraries/src/Installer/Adapter/PackageAdapter.php

@@ -115,15 +115,20 @@ protected function checkExtensionInFilesystem()
      */
     protected function copyBaseFiles()
     {
-        $folder = (string) $this->getManifest()->files->attributes()->folder;
         $source = $this->parent->getPath('source');
 
-        if ($folder) {
-            $source .= '/' . $folder;
+        $attributes = $this->getManifest()->files->attributes();
+
+        if ($attributes) {
+            $folder = (string) $attributes->folder;
+
+            if ($folder) {
+                $source .= '/' . $folder;
+            }
         }
 
         // Install all necessary files
-        if (!\count($this->getManifest()->files->children())) {
+        if (!$this->getManifest()->files->count()) {
             throw new \RuntimeException(
                 Text::sprintf(
                     'JLIB_INSTALLER_ABORT_PACK_INSTALL_NO_FILES',