RFR: 8261711: Clhsdb "versioncheck true" throws NPE every time

[Chris Plummer]

See CR for details. There's no test for `versioncheck`, and I don't think it's worth adding, especially for this trivial fix. Here's how I tested:
hsdb> versioncheck
versioncheck is true
hsdb> versioncheck false
hsdb> versioncheck
versioncheck is false
hsdb> versioncheck true
hsdb> versioncheck
versioncheck is true

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Commit messages:
 - Use System.clearProperty rather than trying to set the property to null.

Changes: https://git.openjdk.java.net/jdk/pull/2566/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2566&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261711
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2566.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2566/head:pull/2566

PR: https://git.openjdk.java.net/jdk/pull/2566