[Bug]: Nil pointer dereference when pushing new datasource to grafana server

[cccs-ajstong]

Grizzly Version

0.4.2

Expected Behavior

When we do a grr push on a datasource that doesn't exist yet on a grafana server, we expect that datasource to be created by grizzly on the grafana server.

Actual Behavior

A nil pointer dereference error happens in grizzly/pkg/grafana/datasource-handler.go line 46 since existing is nil because it is a new datasource (grizzly/pkg/grizzly/workflow.go line 420).

Steps to Reproduce

Just using grr push on a datasource that is not on the grafana server.
grr push datasource.yaml -t Datasource.uid

[malcolmholmes]

This should be fixed by #450.