feat(python): Minor type-inference update for read_database

[alexander-beedie]

Adds basic recognition for "DURATION" and "NULL" type names/strings.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.40%. Comparing base (0c37ead) to head (5950ce2).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15809   +/-   ##
=======================================
  Coverage   80.39%   80.40%           
=======================================
  Files        1264     1264           
  Lines      165421   165423    +2     
=======================================
+ Hits       132994   133004   +10     
+ Misses      32427    32419    -8     

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