Fix one more case of unsetting a key in an object

Author: Seldaek

src/Composer/Json/JsonManipulator.php

@@ -430,6 +430,9 @@ public function removeSubNode(string $mainNode, string $name): bool
             if ($subName !== null) {
                 $curVal = json_decode($matches['content'], true);
                 unset($curVal[$name][$subName]);
+                if ($curVal[$name] === []) {
+                    $curVal[$name] = new \ArrayObject();
+                }
                 $childrenClean = $this->format($curVal, 0, true);
             }
 

tests/Composer/Test/Command/ConfigCommandTest.php

@@ -82,6 +82,11 @@ public static function provideConfigUpdates(): \Generator
             ['setting-key' => 'preferred-install.foo/*', '--unset' => true],
             ['config' => ['preferred-install' => []]],
         ];
+        yield 'unset platform' => [
+            ['config' => ['platform' => ['php' => '7.2.5'], 'platform-check' => false]],
+            ['setting-key' => 'platform.php', '--unset' => true],
+            ['config' => ['platform' => [], 'platform-check' => false]],
+        ];
         yield 'set extra with merge' => [
             [],
             ['setting-key' => 'extra.patches.foo/bar', 'setting-value' => ['{"123":"value"}'], '--json' => true, '--merge' => true],