Reproducing test failure in TestHighFrequencyDictionaryFactory

ant test -Dtestcase=TestHighFrequencyDictionaryFactory -Dtests.method=testDefault -Dtests.seed=9F7E1447BD84930F -Dtests.slow=true -Dtests.locale= -Dtests.timezone=Europe/Oslo -Dtests.asserts=true -Dtests.file.encoding=UTF-8

I suspect this has been around forever, I found it when backporting to 5x and it still reproduces in master.

ERROR 0.00s | TestHighFrequencyDictionaryFactory (suite) <<<
[junit4] > Throwable #1: java.util.IllformedLocaleException: Empty subtag [at index 0]
[junit4] > at __randomizedtesting.SeedInfo.seed([9F7E1447BD84930F]:0)
[junit4] > at java.util.Locale$Builder.setLanguageTag(Locale.java:2426)
[junit4] > at org.apache.lucene.util.LuceneTestCase.localeForLanguageTag(LuceneTestCase.java:1606)
[junit4] > at java.lang.Thread.run(Thread.java:748)
[junit4] Completed [1/1 (1!)] in 2.26s, 0 tests, 1 error <<< FAILURES!
[junit4]