Split OptionalPathSetting from PathSetting

Rather than doing `allowEmpty` as boolean, have separate types and use
`std::optional`. This makes it harder to forget the possibility of an
empty path.

The `build-hook` setting was categorized as a `PathSetting`, but
actually it was split into arguments. No good! Now, it is
`Setting<Strings>` which actually reflects what it means and how it is
used.

Because of the subtyping, we now also have support for
`Setting<std::optional<String>>` in general. I imagine this can be used
to clean up many more settings also.

src/libutil/abstract-setting-to-json.hh

@@ -3,6 +3,7 @@
 
 #include <nlohmann/json.hpp>
 #include "config.hh"
+#include "json-utils.hh"
 
 namespace nix {
 template<typename T>