title: Webpack 5: devtool ValidationError, invalid configuration object
body: <p>While migrating from Webpack 4 to Webpack 5 I got an error when using <code>devtool</code> with empty value (only in production mode).</p>
  <pre><code>module.exports = {
      devtool: isProd ? '' : 'source-map',
      // entry: ...
      // output: ...
      // module: ...
  }
  </code></pre>
  <p>The message in the console:</p>
  <pre><code>ValidationError: Invalid configuration object. Webpack has been initialized using a configuration object that does not match the API schema.
   - configuration.devtool should match pattern &quot;^(inline-|hidden-|eval-)?(nosources-)?(cheap-(module-)?)?source-map$&quot;.
     BREAKING CHANGE since webpack 5: The devtool option is more strict.
     Please strictly follow the order of the keywords in the pattern.
  </code></pre>
  <p>Any ideas how to avoid source maps in production mode? What to type in there?</p>