fix(gatsby-plugin-manifest): Only reassign start_url if it already exists (#21012)
fix(gatsby-plugin-manifest): Only reassign start_url if it already exists (#21012)

* Only reassign start_url if it already exists

Currently, I'm seeing the following error coming from `gatsby-plugin-manifest` when I'm using only the `icon` option:

```
The "path" argument must be of type string. Received type undefined
```

This happens because the `start_url` option is undefined unless set by the user, yet we're passing it as an argument to `path.posix.join`, which expects each argument to be a string. This change adds a condition around the line reassigning `manifest.start_url` to make sure it exists first.

* Fix linting issue

gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby