[ITensors] Fix checkflux for QN ITensor with no blocks

[emstoudenmire]

Fixes #1209 by allowing checkflux to work when block-sparse ITensors are created which have no blocks.

The bug was coming from correlation_matrix due to user-specific operators whose onsite product turned out to be numerically zero (zero matrix), leading to no blocks. This is ok, but the checkflux function was assuming at least one non-zero block and failing in the case of zero blocks.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (15decbd) 85.40% compared to head (198af47) 67.31%.

❗ Current head 198af47 differs from pull request most recent head ca60941. Consider uploading reports for the commit ca60941 to get more accurate results

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1210       +/-   ##
===========================================
- Coverage   85.40%   67.31%   -18.09%     
===========================================
  Files          88       87        -1     
  Lines        8426     8378       -48     
===========================================
- Hits         7196     5640     -1556     
- Misses       1230     2738     +1508     

Files	Coverage Δ
src/qn/flux.jl	78.57% <66.66%> (-13.10%)	⬇️

... and 36 files with indirect coverage changes

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[mtfishman]

Looks good from my end, ready to merge?

[emstoudenmire]

Just missed your comment. Thanks for reviewing. I'll merge.