Change textboxes to display empty instead of 0 when no data

Currently the text boxes will display a "0" if there is no data received. This seems inconsistent with CSS's interpretation, and directly taking a CSS screen that relies on this behaviour will result in a lot of extra 0's being displayed.

See [this comment](https://github.com/dls-controls/cs-web-lib/pull/42#issuecomment-1681864619) for some details of the issue.