compiler: Avoid empty Clusters

[FabioLuporini]

Nothing serious, just a minor oversight triggered by a test in PRO

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.02%. Comparing base (74c6f8e) to head (4b938ea).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
devito/passes/clusters/derivatives.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2820      +/-   ##
==========================================
+ Coverage   83.01%   83.02%   +0.01%     
==========================================
  Files         248      248              
  Lines       51074    51075       +1     
  Branches     4491     4492       +1     
==========================================
+ Hits        42401    42407       +6     
+ Misses       7902     7898       -4     
+ Partials      771      770       -1     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.43% <0.00%> (-0.01%)	⬇️
pytest-gpu-nvc-nvidiaX	68.98% <0.00%> (+0.01%)	⬆️

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