Ignore a postcode tax zone that scores no match

[kha333n]

Fixes #2683.

The same code is unchanged on 2.x, so this wants a forward port.

A customer shipping to the UAE with postcode SW1A 0AA is charged a Sheffield
rate, because the store happens to have a zone on S1 2AB.

The fix

$match is the array ['postcode' => …, 'matches' => 0], which is truthy even
when nothing matched — so the "give up" branch below it was unreachable. The
guard now reads the score inside it, which is the value that says whether the
zone applies at all.

Nothing else changes: the candidate query, the wildcard scoring and the sort are
all untouched, so a zone that does match still wins exactly as before.

Tests

tests/core/Unit/Actions/Taxes/GetTaxZonePostcodeTest.php:

• can not match a postcode that only shares a first character — a store with
  a single S1 2AB zone, asked for SW1A 0AA. Fails on 1.x, which returns the
  S1 2AB row.
• can still match a wildcard when a non-matching zone shares the first
  character — S1 2AB and SW* together; SW* still wins. Passes before and
  after, and is the guard that the fix rejects non-matches rather than weakening
  matching.