[Sitemap v4.1.0] "output" is not allowed to be empty

[mwskwong]

Description

According to PR #31130 and Gatsby v3.5 release notes, it should now be possible to specify output: "" in gatsby-sitemap-plugin@4.1.0. Yet, it appears to reject an empty string value and still report the "output" is not allowed to be empty error, same as older versions.

Steps to reproduce

1. Use any starter template
2. Install gatsby-sitemap-plugin@4.1.0
3. In gatsby-config.js, set the option output to ""
   i.e.

{
  resolve: "gatsby-plugin-sitemap",
  options: {
    output: ""
  }
}

Expected result

gatsby-sitemap-plugin@4.1.0 should accept output: "".

Actual result

It rejected the value and an error is promoted instead.

Environment

  System:
    OS: Windows 10 10.0.18363
    CPU: (12) x64 Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
  Binaries:
    Node: 15.14.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.5 - C:\Program Files (x86)\Yarn\bin\yarn.CMD
    npm: 7.11.1 - C:\Program Files\nodejs\npm.CMD
  Languages:
    Python: 3.9.4
  Browsers:
    Chrome: 90.0.4430.93
    Edge: Spartan (44.18362.449.0)
  npmPackages:
    gatsby: ^3.5.0 => 3.5.0 
    gatsby-plugin-emotion: ^6.5.0 => 6.5.0 
    gatsby-plugin-image: ^1.5.0 => 1.5.0 
    gatsby-plugin-manifest: ^3.5.0 => 3.5.0 
    gatsby-plugin-offline: ^4.5.0 => 4.5.0 
    gatsby-plugin-react-helmet: ^4.5.0 => 4.5.0 
    gatsby-plugin-robots-txt: ^1.6.2 => 1.6.2 
    gatsby-plugin-root-import: ^2.0.6 => 2.0.6 
    gatsby-plugin-sharp: ^3.5.0 => 3.5.0 
    gatsby-plugin-sitemap: ^4.1.0 => 4.1.0 
    gatsby-plugin-webpack-bundle-analyser-v2: ^1.1.22 => 1.1.21 
    gatsby-source-filesystem: ^3.5.0 => 3.5.0 
    gatsby-transformer-sharp: ^3.5.0 => 3.5.0 
  npmGlobalPackages:
    gatsby-cli: 3.0.0

flags: {
  FAST_DEV: true,
  DEV_SSR: true,
  PRESERVE_FILE_DOWNLOAD_CACHE: true,
  PRESERVE_WEBPACK_CACHE: true,
  PARALLEL_SOURCING: true
}

[mwskwong]

Workaround

Set output to "/" or ".".