Isn't a climb by very definition positive?
I think "positive" is used in the sense of "definite" rather than "greater than zero" in this case.
Why over-complicate things? There is no need to start writing a thousand words into your manual to define something which is not an operational problem - you end up with a wordy over-bloated manual, which (because of it's nit-picking definitions) is
more susceptible to bush-lawyer interpretations rather than being simple and clear.
I can't think of a single accident, where the gear has been retracted so early on take-off that it resulted in a problem, so leave it at "positive rate of climb" and forget about it.