Fix time grain

[betodealmeida]

Time grain was being ignored due to a small bug.

[codecov-io]

Codecov Report

Merging #4892 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #4892   +/-   ##
=======================================
  Coverage   76.97%   76.97%           
=======================================
  Files          44       44           
  Lines        8537     8537           
=======================================
  Hits         6571     6571           
  Misses       1966     1966

Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	74.23% <0%> (ø)	⬆️

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