remove empty fields from tiddlers' text serialization

these provide no value and only lead to confusion (cf. #58)

I'm not confident that this all Right - for example, perhaps there's potential for simplification on the corresponding deserialization side, or perhaps even confusion WRT default values or backwards compatiblity. So please review carefully.

Also, feel free to reject this if deemed undesirable.

**updated** to retain empty fields in text serialization except when used by the text store