get_or_create with null values

[dantownsend]

Make sure that get_or_create works correctly with null values.

Fixes #715

[codecov-commenter]

Codecov Report

Merging #716 (0bc2de7) into master (f4e5622) will decrease coverage by 3.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #716      +/-   ##
==========================================
- Coverage   91.21%   88.10%   -3.12%     
==========================================
  Files         106      106              
  Lines        7284     7286       +2     
==========================================
- Hits         6644     6419     -225     
- Misses        640      867     +227     

Impacted Files	Coverage Δ
piccolo/columns/combination.py	94.59% <100.00%> (+0.15%)	⬆️
piccolo/engine/sqlite.py	47.08% <0.00%> (-45.01%)	⬇️
piccolo/columns/defaults/base.py	80.64% <0.00%> (-19.36%)	⬇️
piccolo/query/methods/create_index.py	80.48% <0.00%> (-17.08%)	⬇️
piccolo/query/methods/indexes.py	85.71% <0.00%> (-14.29%)	⬇️
piccolo/apps/sql_shell/commands/run.py	86.11% <0.00%> (-11.12%)	⬇️
piccolo/engine/base.py	80.76% <0.00%> (-9.62%)	⬇️
piccolo/query/methods/select.py	92.21% <0.00%> (-7.38%)	⬇️
piccolo/columns/defaults/timestamp.py	87.27% <0.00%> (-7.28%)	⬇️
piccolo/query/methods/table_exists.py	92.85% <0.00%> (-7.15%)	⬇️
... and 15 more

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