Make _defaultPath return something sensible even if no environment variables are set

Ryunosuke O'Neilrequested to merge
roneil-lbenv-project-path into master

When running inside LHCbDIRAC there are no LbEnv specific environment variables set. This means that functions like findDataPackage always fail.

This MR makes it so that a sensible default is always appended by adding a default value for the lowest priority environment variable.

cc @cburr

Edited by Chris Burr

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