Missing rules Multiplication and Addition node

[bartvanerp]

The Multiplication node misses some rules, which harm the user-friendliness.
These rules include (but might not be limited to)

(*)(::PointMass{<:Real}, ::MultivariateNormalDistributionsFamily)
(*)(::PointMass{<:UniformScaling}, ::MultivariateNormalDistributionsFamily)

[bartvanerp]

Also add

@rule typeof(+)(:in1, ReactiveMP.Marginalisation) (m_out::Missing, m_in2::PointMass) = m_out

@rule typeof(+)(:in1, ReactiveMP.Marginalisation) (m_out::Missing, m_in2::MvNormalMeanCovariance, ) = m_out

@rule typeof(*)(:in, ReactiveMP.Marginalisation) (m_out::Missing, m_A::PointMass, meta::ReactiveMP.TinyCorrection) = m_out

[bvdmitri]

I'm a bit hesitant to add rules with Missing as an argument, because technically it is still a hacky way to pass empty observations and many other parts in ReactiveMP still do not play well with Missings (e.g. joint-marginals, average energy and entropies).

[bvdmitri]

#248 adds rules with missings, do we still need the others?

[bartvanerp]

I think they would still be nice to have