Fix React Vanilla build

Lerna removed the empty dependency attribute of the React Vanilla
package.json during the release. The React Vanilla rollup build
tried to access this attribute and therefore crashed. This is now
fixed.

packages/vanilla/rollup.config.js

@@ -7,7 +7,7 @@ const packageJson = require('./package.json');
 const baseConfig = {
   input: 'src/index.ts',
   external: [
-    ...Object.keys(packageJson.dependencies),
+    ...Object.keys(packageJson.dependencies ?? {}),
     ...Object.keys(packageJson.peerDependencies),
     'react',
     /^lodash\/.*/,