Fix handling of optional parameter description

Jenkins does accept empty parameter description.

Although JJB documents the parameter description as optional,
JJB crashes if no parameter description is provided.

This change fixes the issue by defaulting to an empty string when
no parameter description is provided in a job definition.

Change-Id: Ib848f55007e4863fc8baad3370af650999384524
Merge "Fix handling of optional parameter description"