RFR: 6767: Empty filter regex should not be compiled

[Henrik Dafgård]

This PR allows JMC to short-circuit regex-based IItemFilters that use the empty string to just use a TruePredicate
instead.

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Commit messages:
 - Fix using java blocking rule with empty string

Changes: https://git.openjdk.java.net/jmc/pull/72/files
 Webrev: https://webrevs.openjdk.java.net/jmc/72/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6767
  Stats: 13 lines in 3 files changed: 9 ins; 3 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/72.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/72/head:pull/72

PR: https://git.openjdk.java.net/jmc/pull/72