Clip correlation in dendrogram

[flying-sheep]

• Closes Linkage 'Z' contains negative distances. #2804
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.14%. Comparing base (e6c7251) to head (ea6c7af).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2928      +/-   ##
==========================================
- Coverage   75.48%   73.14%   -2.34%     
==========================================
  Files         116      116              
  Lines       12904    12875      -29     
==========================================
- Hits         9741     9418     -323     
- Misses       3163     3457     +294     

Files	Coverage Δ
scanpy/tools/_dendrogram.py	86.66% <100.00%> (ø)

... and 29 files with indirect coverage changes