What you mean by capture ? If you want dump packet , so for implicit null (label 3) you will never see this label in packet, because this label can be only announced and never pushed in packet. You can find it in output of "show ldp database" or "show mpls lsp extensive" (in Received RRO section) after last hop. "(Label=3)"
For explicit null (label 0) can be captured in packet if "explicit-null" is configured in protocol mpls. For example: