In "hugo config", don't print empty sub maps

[bep]

E.g.

With old style permalinks config like this:

[permalinks]
    post  = "/:year/:month/:slug/"
    photo = "/:year/:month/:slug/"

Running hugo config, you get (something like, I have truncated it a little) this:

[[permalinks]]
  pattern = '/:year/:month/:slug/'
  [permalinks.target]
    path = '/{photo,photo/**}'

    [permalinks.target.sites]
      [permalinks.target.sites.complements]

      [permalinks.target.sites.matrix]

I would expect/prefer this:

[[permalinks]]
  pattern = '/:year/:month/:slug/'
  [permalinks.target]
    path = '/{photo,photo/**}'

The permalinks.target.sites have a IsZero method that should drop it from the output. Not sure why that don't happens.

[algojogacor]

Found the issue — PageMatcher.Sites in resources/page/page_matcher.go is missing a JSON struct tag. Without it, json.Marshal emits the zero-valued {"matrix":{},"complements":{}} even when the field is empty, and the cleanup pass doesn't catch it.

Fix in #14875: adds json:"sites,omitzero" which calls IsZero() during marshal — same pattern already used for the same type in commands/config.go.