Fix missing SNMPv1 instance values

[SoundGoof]

Summary

Return None instead of an empty byte string when an SNMPv1 GET requests a non-existing instance.

This ports the upstream Net-SNMP fix for net-snmp/net-snmp#414, implemented upstream in commit b33d6bbe.

This PR supersedes #9 with a minimal patch based on the current master branch and adds coverage to the active system-test suite.

Root cause

For SNMPv1, a missing instance is reported through err_ind. The response still contains a varbind that was being processed as an OCTETSTR, producing b''.

This is indistinguishable from a valid empty string value.

Changes

• Skip GET response varbind processing at and after the SNMPv1 error index.
• Leave affected tuple entries and Varbind values as None.
• Add an SNMPv1 regression test for a known OID with a non-existing IID.
• Add corresponding SNMPv2c coverage.
• Preserve the existing test for a completely unknown OID.

Behavior

Before:

• SNMPv1 GET for sysDescr.123 returned (b'',).

After:

• SNMPv1 GET for sysDescr.123 returns (None,).
• The affected Varbind.val and Varbind.type remain None.

Testing

• 41 system tests passed.
• ASan/UBSan passed with no reported errors.
• Valgrind passed with:
  • 0 bytes definitely lost
  • 0 bytes indirectly lost
  • 0 bytes possibly lost
  • 0 errors
• git diff --check passed.