Fix show --platform regression failing if no composer.json exists, fixes #11046

src/Composer/Command/ShowCommand.php

@@ -677,7 +677,12 @@ EOT
      */
     protected function getRootRequires(): array
     {
-        $rootPackage = $this->requireComposer()->getPackage();
+        $composer = $this->tryComposer();
+        if ($composer === null) {
+            return [];
+        }
+
+        $rootPackage = $composer->getPackage();
 
         return array_map(
             'strtolower',

tests/Composer/Test/Command/ShowCommandTest.php

@@ -192,6 +192,20 @@ outdated/patch 1.0.0 <highlight>! 1.0.1</highlight>',
         }
     }
 
+    public function testShowPlatformWorksWithoutComposerJson(): void
+    {
+        $this->initTempComposer([]);
+        unlink('./composer.json');
+        unlink('./auth.json');
+
+        $appTester = $this->getApplicationTester();
+        $appTester->run(['command' => 'show', '-p' => true]);
+        $output = trim($appTester->getDisplay(true));
+        foreach (Regex::matchAll('{^(\w+)}m', $output)->matches as $m) {
+            self::assertTrue(PlatformRepository::isPlatformPackage((string) $m[1]));
+        }
+    }
+
     public function testOutdatedWithZeroMajor(): void
     {
         $this->initTempComposer([