MDSMonitor::prepare_command: fix "mds set"

CID 1160828 (#1 of 1): Operands don't affect result (CONSTANT_EXPRESSION_RESULT)
 result_independent_of_operands: !cmd_getval(g_ceph_context, cmdmap,
 std::string("var", std::allocator<char>()), var) < 0 is always false
 regardless of the values of its operands. This occurs as the logical
 operand of if.

Remove '< 0' check, replaced with check if returned string is empty.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>