Description:
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When IntlDateFormatter is created with pattern, getLocale always return empty string. If pattern is not set or set via setPattern - all is OK.

Test script:
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<?php

$goodFormatter = new IntlDateFormatter("ru_RU", IntlDateFormatter::FULL, IntlDateFormatter::FULL, new DateTimeZone("UTC"));
$badFormatter = new IntlDateFormatter("ru_RU", IntlDateFormatter::FULL, IntlDateFormatter::FULL, new DateTimeZone("UTC"), null, "d MMM");
$badFormatter2 = new IntlDateFormatter("ru_RU", IntlDateFormatter::FULL, IntlDateFormatter::FULL, new DateTimeZone("UTC"));
$badFormatter2->setPattern("d MMM");

echo "Formatter without pattern: " . $goodFormatter->getLocale() . PHP_EOL;
echo "Formatter with pattern: " . $badFormatter->getLocale() . PHP_EOL;
echo "Formatter with pattern set later: " . $badFormatter2->getLocale() . PHP_EOL;

Expected result:
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getLocale() always must return original locale.

Actual result:
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Proof - https://3v4l.org/S1CsZ