Nil pointer dereference in FetchArtifact when OCI source has no artifact yet

The sylva-units-operator panics with a nil pointer dereference in pkg/valueshelpers/source_artifacts.go:93 when a SylvaUnitsRelease references an OCI valuesFrom source whose artifact has not yet been downloaded by the flux source-controller.

The panic causes the operator to crash-loop, delaying HelmRelease creation by several minutes until the operator recovers and the OCI source becomes ready.

Error seen in sylvaUnitsOperator logs