fix: do not generate empty file

The recent refactoring in vuejs#4330 introduced a regression where a `Home.vue` file was generated if using the TS plugin even without the router plugin. This happened because the result from the ejs rendering was `undefined`. This fixes the issue by not generating a file if the content is `undefined`.

Author: cexbrayat

Diff for: packages/@vue/cli-plugin-typescript/__tests__/tsGenerator.spec.js

@@ -17,6 +17,8 @@ test('generate files', async () => {
   expect(files['src/main.ts']).toBeTruthy()
   expect(files['src/main.js']).toBeFalsy()
   expect(files['src/App.vue']).toMatch('<script lang="ts">')
+  // checks that the Home.vue file has not be created, even empty
+  expect(files.hasOwnProperty('src/views/Home.vue')).toBeFalsy()
 })
 
 test('classComponent', async () => {
@@ -62,6 +64,22 @@ test('use with Babel', async () => {
   expect(files['tsconfig.json']).toMatch(`"target": "esnext"`)
 })
 
+test('use with router', async () => {
+  const { files } = await generateWithPlugin([
+    {
+      id: '@vue/cli-plugin-router',
+      apply: require('@vue/cli-plugin-router/generator'),
+      options: {}
+    },
+    {
+      id: 'ts',
+      apply: require('../generator'),
+      options: {}
+    }
+  ])
+  expect(files['src/views/Home.vue']).toMatch('<div class=\"home\">')
+})
+
 test('lint', async () => {
   const { pkg, files } = await generateWithPlugin([
     {

Diff for: packages/@vue/cli/lib/GeneratorAPI.js

@@ -244,7 +244,7 @@ class GeneratorAPI {
           const sourcePath = path.resolve(source, rawPath)
           const content = renderFile(sourcePath, data, ejsOptions)
           // only set file if it's not all whitespace, or is a Buffer (binary files)
-          if (Buffer.isBuffer(content) || /[^\s]/.test(content)) {
+          if (Buffer.isBuffer(content) || content && /[^\s]/.test(content)) {
             files[targetPath] = content
           }
         }
@@ -255,7 +255,7 @@ class GeneratorAPI {
         for (const targetPath in source) {
           const sourcePath = path.resolve(baseDir, source[targetPath])
           const content = renderFile(sourcePath, data, ejsOptions)
-          if (Buffer.isBuffer(content) || content.trim()) {
+          if (Buffer.isBuffer(content) || content && content.trim()) {
             files[targetPath] = content
           }
         }