net-snmp: add early return when HostName is unset

net-snmp: add early return when HostName is unset

snmpd_access_HostName_add would write unintended output to the
config file if the HostName option is not specified. Add an early
return to prevent writing in that case.

This is a breaking change for configs where HostName is unset.
Previously, an empty HostName option would result in a malformed
directive with a trailing 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>