[avalanche] Don't reject orphan txs

Summary: Orphan txs (i.e. txs missing inputs) are not necessarily invalid, we might just be missing the parents. Don't vote against such txs but rather vote "I don't know".

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_transaction_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15361