Add check for zero norm to expect

[emstoudenmire]

Description

Currently if you pass an MPS to expect whose norm is zero, the code will compute the norm and later divide by it, siletly producing NaN's in the output. A user recently ran into this issue. This PR adds a simple error check of the norm.

Minimal demonstration of previous behavior

using ITensors

function ITensors.op!(Op::ITensor,
                      ::OpName"Pdn",
                      ::SiteType"S=1/2",
                      s::Index)
  Op[s'=>2,s=>2] = 1.0
end

let
  N = 10
  s = siteinds("S=1/2",N)

  psi0 = MPS(s,"Up")
  psi1 = apply(op("Pdn",s[5]),psi0)

  @show expect(psi1,"Sz")

  return
end

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d9265a2) 85.44% compared to head (b8d3e35) 54.80%.
Report is 2 commits behind head on main.

❗ Current head b8d3e35 differs from pull request most recent head 2767746. Consider uploading reports for the commit 2767746 to get more accurate results

Files	Patch %	Lines
src/mps/mps.jl	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1249       +/-   ##
===========================================
- Coverage   85.44%   54.80%   -30.64%     
===========================================
  Files          89       88        -1     
  Lines        8401     8349       -52     
===========================================
- Hits         7178     4576     -2602     
- Misses       1223     3773     +2550     

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[mtfishman]

Makes sense, thanks! Could you add a test?