lib.modules: submodule emptyValue must evaluate sub-option defaults

`emptyValue` for `types.submodule` was `{ value = { }; }`, i.e. none
of the type-declared options or their defaults.

Previously submodules without defs would simply fail for lack of a default,
but since PR #500104 repurposed `emptyValue` as the fallback when no
definitions exist, submodule options without an explicit `default = { }`
(new additions) silently lost their sub-option defaults
(e.g. `requiredFeatures.devnet` in the nixos-test-driver, #511413).

Main change: `emptyValue`: `{ }` -> `{ value = base.config; }`

Additionally, skip `emptyValue`-based default rendering in docs for
types with submodules, because their sub-options are already documented
individually, and forcing evaluation here can break on modules with
invalid or incomplete definitions.

lib/types.nix

@@ -1320,7 +1320,7 @@ rec {
           };
       };
       emptyValue = {
-        value = { };
+        value = base.config;
       };
       getSubOptions =
         prefix: