pkg/sensors: fix a marshalling mistake on the error field

Previously, we were using fmt.Sprint(err) that was printing <nil> on nil
error, using err.Error() and setting the field only if the error is not
nil is more appropriate.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>

pkg/sensors/handler.go

@@ -171,10 +171,13 @@ func (h *handler) listTracingPolicies(op *tracingPolicyList) error {
 			Name:     name,
 			Enabled:  col.state == EnabledState,
 			FilterId: col.policyfilterID,
-			Error:    fmt.Sprint(col.err),
 			State:    col.state.ToTetragonState(),
 		}
 
+		if col.err != nil {
+			pol.Error = col.err.Error()
+		}
+
 		pol.Namespace = ""
 		if tpNs, ok := col.tracingpolicy.(tracingpolicy.TracingPolicyNamespaced); ok {
 			pol.Namespace = tpNs.TpNamespace()