wesnothd/fuh: Fix instances of std::string initialized from time_t(0)

Fixes the build with the fuh enabled on clang 3.3:

> src/server/forum_user_handler.cpp:206:10: error: expression which evaluates to zero treated as a null pointer constant of type 'const char *' [-Werror,-Wnon-literal-null-conversion]
>   return time_t(0);
>          ^~~~~~~~~
> src/server/forum_user_handler.cpp:215:10: error: expression which evaluates to zero treated as a null pointer constant of type 'const char *' [-Werror,-Wnon-literal-null-conversion]
>   return time_t(0);
>          ^~~~~~~~~

A time_t(0) is cast equivalently to a null pointer, and an empty C
string is equivalent to a null pointer for std::string instantiation.