rgw: fix setting of NULL to string

Fixes: #3777
s->env->get() returns char * and not string and can return NULL.
Also, remove some old unused code.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit 9019fbbe8f84f530b6a8700dfe99dfeb03e0ed3d)