Default exec command to use IPFS

A publisher is required when output folders are specified, otherwise we
will get a validation error (now that generally no publisher is required).

cmd/cli/exec/exec.go

@@ -222,6 +222,11 @@ func PrepareJob(cmd *cobra.Command, cmdArgs []string, unknownArgs []string, opti
 			Type:   publisherSpec.Type.String(),
 			Params: publisherSpec.Params,
 		}
+	} else {
+		job.Tasks[0].Publisher = &models.SpecConfig{
+			Type:   "ipfs",
+			Params: map[string]interface{}{},
+		}
 	}
 
 	// Handle ResultPaths by using the legacy parser and converting.
@@ -309,6 +314,10 @@ func prepareJobOutputs(ctx context.Context, options *ExecOptions, job *models.Jo
 		return err
 	}
 
+	if len(legacyOutputs) == 0 {
+		return nil
+	}
+
 	job.Tasks[0].ResultPaths = make([]*models.ResultPath, 0, len(legacyOutputs))
 	for _, output := range legacyOutputs {
 		rp := &models.ResultPath{