WS-677: `null_dag` capability

Additions

• Added new capability: null_dag that simply launches the null_dag workflow
• Added button to launch new capability to the sandbox
• Changed "no-files" message to "No files are attached to this request."; added "no-parameters" message: "This request has no parameters."

Screenshot

New button

New empty-parameters/files messages

Completes WS-677 (for real this time)