net-snmp: add early return when Mode is unset

net-snmp: add early return when Mode is unset

snmpd_access_default_add, snmpd_access_HostName_add, and
snmpd_access_HostIP_add would write unintended output to the
config file if the Mode option is not specified. Add an early
return to each function to prevent writing in that case.

This is a breaking change for configs where Mode is unset.
Previously, an empty Mode option would result in a malformed
directive with a leading space and no guaranteed newline, the
exact form of which depended on the values of other options.
Now, no directive is written.

Signed-off-by: Eric McDonald <redacted>