Fix the issue with plugin installation from npm

Summary: https://github.com/facebook/flipper/issues/1818

Reviewed By: jknoxville

Differential Revision: D25870562

fbshipit-source-id: 0d62626355b35505089990cfdb592b0ea4b84fda

desktop/plugin-lib/src/__tests__/pluginInstaller.node.ts

@@ -147,6 +147,12 @@ describe('pluginInstaller', () => {
     ]);
   });
 
+  test('getInstalledPlugins when no plugins installed', async () => {
+    await fs.remove(pluginInstallationDir);
+    const plugins = await getInstalledPlugins();
+    expect(plugins).toHaveLength(0);
+  });
+
   test('moveInstalledPluginsFromLegacyDir', async () => {
     await moveInstalledPluginsFromLegacyDir();
     expect(