[FIX] SqlTable use empty string instead of None for StringVariable

[PrimozGodec]

Issue

• SQL table places None where missing value for StringVariable. It should use '' (empty string) instead. It breaks merge data widget with does not expect None in the String variable.
• SQL table tests do not cleanup the database after testing.

Description of changes

• Removing SqlRowInstance and using Instance instead since it seems that Instance does the same in the __init__ that SqlRowInstance used to do. It also correctly replaces None with the '' (empty string). Hope I didn't missee anything.
• Remove the database tables after testing

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5120 (6804210) into master (ef789ee) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5120      +/-   ##
==========================================
- Coverage   84.77%   84.77%   -0.01%     
==========================================
  Files         286      286              
  Lines       60084    60078       -6     
==========================================
- Hits        50938    50929       -9     
- Misses       9146     9149       +3