CAS-1397: Add null checks for get/put ops of ClearPass

Original pull request: https://github.com/Jasig/cas/pull/371

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No logging implemented for handling possible decoratedMap exceptions.

For example, using spymemcached specified in Spring configuration for clearpass-configuration.xml could throw OperationTimeoutException. See http://stackoverflow.com/q/10800101/901156

JIRA: https://issues.jasig.org/browse/CAS-1397

This is also related to JIRA https://issues.jasig.org/browse/CAS-1394 as we are looking to propose these changes because we are still getting an empty pgtIou and this serves as means of adding debugging capabilities for spymemcached implementations.