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guard: Don't check bridge transport name when selecting eligible guards

This is related to ticket #40360 which found this problem when a Bridge entry
with a transport name (let say obfs4) is set without a fingerprint:

  Bridge obfs4 <IP>:<PORT> cert=<...> iat-mode=0

(Notice, no fingerprint between PORT and "cert=")

Problem: commit 09c6d0324 added a check in
get_sampled_guard_for_bridge() that would return NULL if the selected bridge
did not have a valid transport name (that is the Bridge transport name that
corresponds to a ClientTransportPlugin).

Unfortuantely, this function is also used when selecting our eligible guards
which is done *before* the transport list is populated and so the added check
for the bridge<->transport name is querying an empty list of transports
resulting in always returning NULL.

For completion, the logic is: Pick eligible guards (use bridge(s) if need be)
then for those, initiate a connection to the pluggable transport proxy and
then populate the transport list once we've connected.

Back to get_sampled_guard_for_bridge(). As said earlier, it is used when
selecting our eligible guards in a way that prevents us from selecting
duplicates. In other words, if that function returns non-NULL, the selection
continues considering the bridge was sampled before. But if it returns NULL,
the relay is added to the eligible list.

This bug made it that our eligible guard list was populated with the *same*
bridge 3 times like so (remember no fingerprint):

  [info] entry_guards_update_primary(): Primary entry guards have changed. New primary guard list is:
  [info] entry_guards_update_primary():   1/3: [bridge] ($<a class=hub.com/torproject/tor/commit/<a class="double-link" href="https://git"><a class=hub.com/torproject/tor/commit/0000000000000000000000000000000000000000">000000000)
  [info] entry_guards_update_primary():   2/3: [bridge] ($0000000000000000000000000000000000000000)
  [info] entry_guards_update_primary():   3/3: [bridge] ($0000000000000000000000000000000000000000)

When tor starts, it will find the bridge fingerprint... (continued)

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