Zero length Teaser lost when field re-edited

In some cases I wish have a zero-length teaser; e.g. in a given View I want some nodes to show a teaser of (say) a paragraph, but on other nodes only the Title should show. This is an editorial decision, and not built into the design.

If I put a Teaser Break at the start of the field, this is fine - it does what I want. (It is a bit awkward putting a Teaser Break right at the start, but OK once one has worked it out.) However the next time one edits that node the Teaser Break has disappeared. It looks as though FCKEditor is saying (deliberately or otherwise) "Oh there is no teaser on this field, I will throw the break away", which is not what is wanted.

With thanks, Tony.