fixing another issue likely with pandas update: "None" items in the l…

[chenyangkang]

…ist passed to the pd.concat will be ignore, so passing an empty dataframe instead of None for the ensembles which have all np.nan predictions.

Continued for #87, related to #88 and #90

[codecov]

Codecov Report

❌ Patch coverage is 98.14815% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.72%. Comparing base (b071dcc) to head (3e0ea15).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
stemflow/model/AdaSTEM.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
- Coverage   90.75%   90.72%   -0.03%     
==========================================
  Files          40       40              
  Lines        3634     3634              
==========================================
- Hits         3298     3297       -1     
- Misses        336      337       +1     

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