Let pipeline handle null for optional values
Let pipeline handle null for optional values

- makes optional values compatible with ((interpolation))

[#153355053](https://www.pivotaltracker.com/story/show/153355053)

Signed-off-by: Felix Riegger <9c2aa7530239a330bbac6ac4f56e0946781dfd29@sap.com>

cloudfoundry-incubator/bosh-openstack-cpi-release,cloudfoundry-incubator/bosh-openstack-cpi-release,cloudfoundry-incubator/bosh-openstack-cpi-release