Config CLI: Handle missing config values correctly

If the domain/group/key doesn't exist in the config, exit with
non-zero status and don't print out anything.

Previously the CLI would print a single empty line if the config
value was not found with LibConfig. Now, we use the proper
`Config::Client::the().read_string()` API which can return an
`Optional` type indicating failure.`

Userland/Utilities/config.cpp

@@ -29,8 +29,9 @@ int main(int argc, char** argv)
  return 0;
  }
 
- auto value = Config::read_string(domain, group, key);
- outln("{}", value);
-
+ auto value_or_error = Config::Client::the().read_string_value(domain, group, key);
+ if (!value_or_error.has_value())
+ return 1;
+ outln("{}", value_or_error.value());
  return 0;
 }