Improve fix in r1626998 (itself an improvement on the fix for POOL-161)
Handle the case where the developer has explicitly set the TCCL to null during the construction of the pool.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/pool/trunk@1627270 13f79535-47bb-0310-9956-ffa450edef68