Support missing values in dot

[nalimilan]

This is more consistent with matrix product, which propagates missing automatically since * and + are defined for it.

This is needed to avoid a regression in Statistics.cov in Julia 1.7 compared with 1.6. https://github.com/JuliaLang/Statistics.jl/pull/85 replaced a call to sum(conj(y[i])*x[i] for i in eachindex(y, x)) with dot(y, x), which is numerically more stable and was originally used before Statistics was moved to the stdlib (at the time LinearAlgebra was still in Base so dot could not be used from an stdlib). But this change currently makes cov throw an error when the input contains missing, which was not the case in Julia 1.6. This currently makes StatsBase tests fail on Julia 1.7.

EDIT: a solution in Statistics is to use adjoint(y) * x instead of dot(y, x), as the former falls back to the latter for arrays of Numbers but to sum(uu*vv for (uu, vv) in zip(u, v)) for other types. See https://github.com/JuliaLang/Statistics.jl/pull/94. Though it can still be useful to support missing values with dot.

[dkarrasch]

x-ref #40769 🤣