Ensure pass string on PathNormalizer::normalize() on InstalledPackageResolver (#6865)

* Ensure pass string on PathNormalizer::normalize() on InstalledPackageResolver

* clean up

Author: samsonasik

src/Composer/InstalledPackageResolver.php

@@ -84,14 +84,17 @@ private function resolveVendorDir(): string
             if (isset($projectComposerJson['config']['vendor-dir']) &&
                 is_string($projectComposerJson['config']['vendor-dir'])
             ) {
-                return PathNormalizer::normalize(
-                    realpath($projectComposerJson['config']['vendor-dir'])
-                ) === PathNormalizer::normalize($projectComposerJson['config']['vendor-dir'])
+                $realPathVendorDir = realpath($projectComposerJson['config']['vendor-dir']) ?: '';
+                $normalizedRealPathVendorDir = PathNormalizer::normalize($realPathVendorDir);
+                $normalizedVendorDir = PathNormalizer::normalize($projectComposerJson['config']['vendor-dir']);
+
+                return $normalizedRealPathVendorDir === $normalizedVendorDir
                     ? $projectComposerJson['config']['vendor-dir']
                     : $this->projectDirectory . '/' . $projectComposerJson['config']['vendor-dir'];
             }
 
         }
+
         return $this->projectDirectory . '/vendor';
     }
 }