misc: Fix operator output with OI=None

[FabioLuporini]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.29%. Comparing base (78691c5) to head (eba86d9).
Report is 2 commits behind head on master.

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2495      +/-   ##
==========================================
- Coverage   87.29%   87.29%   -0.01%     
==========================================
  Files         238      238              
  Lines       45379    45380       +1     
  Branches     4029     4030       +1     
==========================================
  Hits        39615    39615              
  Misses       5083     5083              
- Partials      681      682       +1     

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