Account for BP edge case where network evaluates to 0

[JoeyT1994]

This PR fixed the bp edge case where the network contracts to the scalar 0 which would previously yield Nan.

This is achieved by:

1. Updating the default_message_update function to only divide by the norm if its not zero -> avoiding the creating of message tensors with nan in them.
2. Adding to logscalar(alg::Algorithm"bp", tn::AbstractITensorNetwork) a check if any of the denominators terms are zero. If this is the case then log(tn) should be -Inf. This is because, on a tree, if the product of message(pe) and message(reverse(pe) on any partitionedge is zero then the contraction of the tree must be zero (assuming the messages are converged).

A quick test is added.

@ryanlevy this should fix the issue with bp in ITensorNumericalAnalysis.jl