To think about this, you need to consider the meaning.
The form "Shall we ..." is used to make a suggestion of an activity. In the affirmative, this makes sense. You suggest a action. "Shall we play tennis"
What would the negative mean. Well, not making a suggestion is the same as doing nothing. That isn't a sensible thing to describe.
It is possible to suggest not doing something. In that case the "not" is applied to the verb in the infintive clause. "Shall we not play tennis". But this is odd. You see how can you tell the difference between "not playing tennis" and "not playing football"!? It would nearly always be better to positively suggest the alternative "We always play tennis on Sunday. This weekend, shall we go for a walk together instead?" It is better to say "shall we go for a walk" instead of "Shall we not play tennis."
Finally there is the possibility of the negative question. Negative questions carry a strong implication (compare the neutral "can you play tennis" with the marked "can't you play tennis" - the latter means that the speaker would be surprised that you can't) But this implication doesn't work well with "Shall we" type suggestions.
In conclusion, English speakers would tend not to use a negative form of "shall we".