StringHelper should check for empty string of "tests.seed" system property [LUCENE-5645]

As of #6666 (committed to v4.8), StringHelper will initialize GOOD_FAST_HASH_SEED based on the system property "tests.seed" if it is set. Unfortunately it doesn't do an empty-string check, and it's common at least in my setup that copies Lucene's maven pom.xml that the string will be empty unless I set it on the command line. FWIW Randomized Testing does do an empty-string check.

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Migrated from [LUCENE-5645](https://issues.apache.org/jira/browse/LUCENE-5645) by David Smiley (@dsmiley), 1 vote, updated May 09 2016
Attachments: [LUCENE-5645_StringHelper_empty_tests_seed_condition.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5645/LUCENE-5645_StringHelper_empty_tests_seed_condition.patch)