* fix none-ness for string inference

[realratchet]

Inferring a string type as string should not become a none.

[codecov-commenter]

Codecov Report

Merging #82 (a1e0e49) into master (3c79d3f) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   89.40%   89.39%   -0.02%     
==========================================
  Files          49       49              
  Lines        6761     6763       +2     
==========================================
+ Hits         6045     6046       +1     
- Misses        716      717       +1     

Flag	Coverage Δ
pytest	89.39% <100.00%> (-0.02%)	⬇️

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Files Changed	Coverage Δ
tablite/datatypes.py	77.44% <100.00%> (-0.12%)	⬇️
tablite/version.py	100.00% <100.00%> (ø)

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