fix(runtime-core): fix error when passed plugin is undefined (#502)

packages/runtime-core/__tests__/apiApp.spec.ts

@@ -244,6 +244,7 @@ describe('api: createApp', () => {
     const PluginB: Plugin = {
       install: app => app.provide('bar', 2)
     }
+    const PluginC: any = undefined
 
     const app = createApp()
     app.use(PluginA)
@@ -264,6 +265,12 @@ describe('api: createApp', () => {
     expect(
       `Plugin has already been applied to target app`
     ).toHaveBeenWarnedTimes(1)
+
+    app.use(PluginC)
+    expect(
+      `A plugin must either be a function or an object with an "install" ` +
+        `function.`
+    ).toHaveBeenWarnedTimes(1)
   })
 
   test('config.errorHandler', () => {