(fix): correct shape for empty element in concat with dask

[ilan-gold]

• Closes missing dask-elem imputation for concat not working with differently sized input objects #1842
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.69%. Comparing base (3d0105b) to head (8b2e208).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/_core/merge.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1843      +/-   ##
==========================================
- Coverage   86.13%   83.69%   -2.45%     
==========================================
  Files          41       41              
  Lines        6277     6282       +5     
==========================================
- Hits         5407     5258     -149     
- Misses        870     1024     +154     

Files with missing lines	Coverage Δ
src/anndata/_core/merge.py	82.86% <81.81%> (-11.10%)	⬇️

... and 7 files with indirect coverage changes