fix: have the bundle.js generated for SSR on the server

[RossComputerGuy]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Couldn't find the original issue but this fixes the problem of the server side webpack bundle.js not being generated. I made the fix based on how the comment was made so it's a relatively simple fix.

[posva]

Can you please provide a reproduction?

[RossComputerGuy]

When trying to build my website, https://github.com/emberliteOS/website/, only the client bundle js is generated and not the server's. This PR should fix that.

[posva]

But a boiled down reproduction we can take a look at. Or a test that failed before (not sure if this one is feasible that way)

[posva]

Closing due to inactivity. Please open a new issue with a reference to this one if you can follow up with more information.