net-snmp: fix literal string test in snmpd_sink_add

net-snmp: fix literal string test in snmpd_sink_add

The function snmpd_sink_add() has a guard clause that tests the literal
string "section", not the variable value "$section".

The test `[ -n "section" ]` always evaluates to true because the string
literal "section" is non-empty, making the check useless.

This function is only called internally with hardcoded arguments, so the
bug has no actual impact currently. For the same reason, this change
should not break existing configurations. However, I think it should be
fixed so future callers do not have a false sense of security.

Signed-off-by: Eric McDonald <librick-openwrt@proton.me>