refactor(topology/algebra/infinite_sum): generalize `tsum_zero` (#15786)
refactor(topology/algebra/infinite_sum): generalize `tsum_zero` (#15786)

Thanks to @kbuzzard and @b-mehta, it holds whenever
`is_closed {0}`. This is true not just as `t2_space` as before,
but in all `t1_space`.







leanprover-community/mathlib,leanprover-community/mathlib