net-snmp: add early return when CommunityName is unset

net-snmp: add early return when CommunityName 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 CommunityName 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 CommunityName is
unset. Previously, an empty CommunityName option would result in
a malformed directive with a double 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>