The reason I imagine it is not working is you have this textbox in a data (non-header) row of the tablix. Since there is no data, this row will be repeated exactly zero times.
You should be able to do either of the following:
Add a header row to the table with the text "NO DATA FOUND" which is hidden on
=Count(Fields!ContentId.Value) > 0
.Add a normal textbox outside of the tablix below the table, similar to the above. You may need to explicitly specify the data source. If it is hidden, the space should be consumed and the report will display as normal.