fs/AllocatedPath: make the nullptr_t constructor public

src/storage/CompositeStorage.cxx

@@ -316,7 +316,7 @@ CompositeStorage::MapFS(const char *uri) const noexcept
 
 	auto f = FindStorage(uri);
 	if (f.directory->storage == nullptr)
-		return AllocatedPath::Null();
+		return nullptr;
 
 	return f.directory->storage->MapFS(f.uri);
 }

src/storage/StorageInterface.cxx

@@ -25,7 +25,7 @@
 AllocatedPath
 Storage::MapFS(gcc_unused const char *uri_utf8) const noexcept
 {
-	return AllocatedPath::Null();
+	return nullptr;
 }
 
 AllocatedPath

src/storage/StorageInterface.hxx

@@ -66,7 +66,7 @@ public:
 
 	/**
 	 * Map the given relative URI to a local file path.  Returns
-	 * AllocatedPath::Null() on error or if this storage does not
+	 * nullptr on error or if this storage does not
 	 * support local files.
 	 */
 	gcc_pure

src/storage/plugins/LocalStorage.cxx

@@ -124,7 +124,7 @@ LocalStorage::MapFS(const char *uri_utf8) const noexcept
 	try {
 		return MapFSOrThrow(uri_utf8);
 	} catch (...) {
-		return AllocatedPath::Null();
+		return nullptr;
 	}
 }