BUG: pick_connection() breaks when looking up non-existing connections

[ntolley]

Found by @chenghuzi in #500. There is a flaw in the logic of pick_connection() where searching connections that don't exist in the Network returns a non-empty list. Easy example to reproduce:

from hnn_core import jones_2009_model, pick_connection
net = jones_2009_model()

src_gids = 'L2_basket'
target_gids = 'L2_basket'
loc = 'proximal'
receptor = 'ampa'
conn_indices = pick_connection(
    net, src_gids=src_gids, target_gids=target_gids, loc=loc, receptor=receptor)

print(conn_indices)

[1, 3, 8, 9, 11, 14, 15]

If you look up these connections they are actually all the ones that contain ampa synapses, which indicates the problem with how pick_connection searches through net.connectivity. Will open a PR to fix soon.