misc: fix UnboundTuple for None partile

[mloubout]

Fix broken 2.5d tile

[codecov]

Codecov Report

Merging #2256 (b86d0b4) into master (465e72b) will increase coverage by 0.00%.
The diff coverage is 85.29%.

@@           Coverage Diff           @@
##           master    #2256   +/-   ##
=======================================
  Coverage   86.94%   86.95%           
=======================================
  Files         229      229           
  Lines       41880    41919   +39     
  Branches     7733     7740    +7     
=======================================
+ Hits        36414    36451   +37     
+ Misses       4832     4831    -1     
- Partials      634      637    +3     

Files	Coverage Δ
devito/arch/compiler.py	40.00% <ø> (ø)
devito/core/operator.py	84.36% <100.00%> (ø)
devito/passes/iet/parpragma.py	88.80% <100.00%> (ø)
tests/test_tools.py	100.00% <100.00%> (ø)
tests/test_unexpansion.py	100.00% <100.00%> (ø)
devito/passes/iet/languages/openacc.py	63.80% <0.00%> (ø)
devito/passes/clusters/blocking.py	88.80% <71.42%> (-0.65%)	⬇️
devito/tools/data_structures.py	68.66% <75.00%> (+1.12%)	⬆️