Fix PostCSS crash when using toJSON()

[thecrypticace]

When I added source maps to PostCSS I mistakenly assumed that .source on a node could be undefined. The comment above the property in PostCSS says that source can be undefined but this is a commentary on the value upon access not its expected value on write:

declare abstract class Node_ {
  /**
   * …
   *
   * The nodes that are created manually using the public APIs
   * provided by PostCSS will have `source` undefined and
   * will be absent in the source map.
   *
   * …
   */
  source?: Node.Source
}

Rather, what these types mean is that if the property exists it must be defined. But otherwise the property can be missing if a node has no source location metadata. This generally wasn't a problem with the string-returning APIs but the toJSON() API in PostCSS expects that source is defined if present. This caused a crash because our license comment doesn't have a source location.

I've addressed this by deleting the source property from the node if source location data is not available.

Fixes #18082

ref parcel-bundler/parcel#10161