Fixed Model.import_data for empty dict

[altvod]

If you call Model.import_dict() with a dictionary that is created by some other part of your program, it might be empty, but the import_dict method should still work the same way as it would if the dictionary had any values in it, i.e. convert only the incoming data (of which there isn't any). Otherwise it leads to data being converted twice, which is unacceptable as it can cause corruption of the stored data.

[codecov-io]

Codecov Report

Merging #584 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #584   +/-   ##
=======================================
  Coverage   91.06%   91.06%           
=======================================
  Files          24       24           
  Lines        2541     2541           
=======================================
  Hits         2314     2314           
  Misses        227      227

Impacted Files	Coverage Δ
schematics/models.py	97.88% <100%> (ø)	⬆️

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