fix(GeneratorAPI): remove warning when using extendPackage with prune (#5202)

Currently, if we use `api.extendPackage({foo: null}, {prune: true})` the dependency is removed but a warning is logged as the range is invalid:

```
 WARN  invalid version range for dependency "foo":

- null injected by generator "my-plugin"
```

Author: cexbrayat

packages/@vue/cli/__tests__/Generator.spec.js

@@ -457,6 +457,9 @@ test('api: extendPackage + { prune: true }', async () => {
 
   await generator.generate()
 
+  // should not warn about the null versions
+  expect(logs.warn.length).toBe(0)
+
   const pkg = JSON.parse(fs.readFileSync('/package.json', 'utf-8'))
   expect(pkg).toEqual({
     version: '0.0.0',

packages/@vue/cli/lib/util/mergeDeps.js

@@ -42,6 +42,7 @@ module.exports = function mergeDeps (
 
     if (prune && injectingRange == null) {
       delete result[depName]
+      continue
     }
 
     if (!isValidRange(injectingRange)) {