quantile! produces an unrelated assertion error when computing quantiles of a large range

[yurivish]

In an earlier issue @jishnub noticed that certain ranges are considered nonempty despite the fact that their length is zero:

julia> r = typemin(Int):typemax(Int)
-9223372036854775808:9223372036854775807

julia> length(r)
0

julia> isempty(r)
false

This behavior triggers an assertion error in quantile! even though quantiles for such ranges are mathematically well-defined:

julia> using Statistics

julia> quantile!([0], r, [1])
ERROR: AssertionError: n > 0

The assertion has the comment "this case should never happen here":

Statistics.jl/src/Statistics.jl

Line 994 in 0588f2c

@assert n > 0 # this case should never happen here