Fix tests

tests/Composer/Test/Fixtures/installer/install-dev-using-dist.test

@@ -41,7 +41,7 @@ install --prefer-dist
             "type": "library"
         }
     ],
-    "packages-dev": null,
+    "packages-dev": [],
     "aliases": [],
     "minimum-stability": "dev",
     "stability-flags": {

tests/Composer/Test/Fixtures/installer/update-alias-lock.test

@@ -60,7 +60,7 @@ update
             "type": "library"
         }
     ],
-    "packages-dev": null,
+    "packages-dev": [],
     "aliases": [],
     "minimum-stability": "dev",
     "stability-flags": [],

tests/Composer/Test/Fixtures/installer/updating-dev-updates-url-and-reference.test

@@ -53,7 +53,7 @@ update
             "dist": { "reference": "newref", "url": "newurl", "type": "zip", "shasum": "" }
         }
     ],
-    "packages-dev": null,
+    "packages-dev": [],
     "aliases": [],
     "minimum-stability": "dev",
     "stability-flags": {"a/a":20},