There is not much you can do about that effect. The text properties are tied to actual text, and therefore, you will have to keep some text in the field. The easiest you can keep in the field would be a properly formatted space character.
a) the richText property of text fields, as well as richValue has NOTHING to do with the .rtf file format. It is therefore a bit dangerous to call fields with richText set "RTF fields". Unfortunately, Adobe lacked the according quality control with the localization, and at least in German, they talk about RTF.
b) Fields with richText set are not reset-proof. There is no richDefaultValue property, and therefore you would lose all contents when you reset the form; you will have to provide your own method to refill fields with richText set after a reset.
Hope this can help.