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.

src/server/forum_user_handler.cpp

@@ -205,7 +205,7 @@ std::string fuh::get_hash(const std::string& user) {
 		return get_detail_for_user(user, "user_password");
 	} catch (error& e) {
 		ERR_UH << "Could not retrieve password for user '" << user << "' :" << e.message << std::endl;
-		return time_t(0);
+		return "";
 	}
 }
 
@@ -214,7 +214,7 @@ std::string fuh::get_mail(const std::string& user) {
 		return get_detail_for_user(user, "user_email");
 	} catch (error& e) {
 		ERR_UH << "Could not retrieve email for user '" << user << "' :" << e.message << std::endl;
-		return time_t(0);
+		return "";
 	}
 }