Fix JENKINS-20660 - null pointer exception on first run after upgrade 0.8 to 0.9

Jobs which were defined with an XShell builder prior to 0.9 will report a null pointer exception and fail the first time they are run with the 0.9 version of the plugin.  The plugin assumes the string arguments to the constructor will never be null, yet in the upgrade case they are null (rather than empty strings).

This does not resolve any latent issues with a null timeAllocated, though that seems to be handled already (possibly unintentionally) by trapping all exceptions thrown during the parsing of timeAllocated, including a null pointer exception.