RFR: 8217496: Matcher.group() can return null after usePattern

[Ian Graves]

Specification update to clarify Matcher behavior to include a null return value.

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Commit messages:
 - 8217496 Matcher.group() can return null after usePattern

Changes: https://git.openjdk.java.net/jdk/pull/5827/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5827&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8217496
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5827.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5827/head:pull/5827

PR: https://git.openjdk.java.net/jdk/pull/5827