use .toString() instead of casting to String in StringType

[knaeckeKami]

This solves the problem of columns with mixed types as described in #69

[simolus3]

Looks good with the nullable call, thanks for your contribution 👍

[codecov-io]

Codecov Report

Merging #70 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           develop    #70      +/-   ##
=========================================
+ Coverage    86.99%    87%   +<.01%     
=========================================
  Files           69     69              
  Lines         1938   1939       +1     
=========================================
+ Hits          1686   1687       +1     
  Misses         252    252

Impacted Files	Coverage Δ
moor/lib/src/types/sql_types.dart	80% <100%> (+0.58%)	⬆️

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