fix JSONB column where null

[dantownsend]

Related to #410

For example:

class RecordingStudio(Table):
    facilities = JSONB(null=True)

# This was causing an error:
await RecordingStudio.select().where(
    RecordingStudio.facilities.is_null()
)

The problem was due to a bug with how Piccolo serialises JSON values.

[codecov-commenter]

Codecov Report

Merging #413 (3164197) into master (c15eb0e) will decrease coverage by 0.03%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #413      +/-   ##
==========================================
- Coverage   90.87%   90.84%   -0.04%     
==========================================
  Files         103      103              
  Lines        6850     6867      +17     
==========================================
+ Hits         6225     6238      +13     
- Misses        625      629       +4     

Impacted Files	Coverage Δ
piccolo/columns/base.py	95.16% <ø> (ø)
piccolo/columns/column_types.py	87.05% <71.42%> (-0.17%)	⬇️
piccolo/apps/playground/commands/run.py	38.88% <100.00%> (ø)
piccolo/columns/combination.py	94.52% <100.00%> (+0.15%)	⬆️
piccolo/query/base.py	85.50% <100.00%> (+0.14%)	⬆️
piccolo/utils/sql_values.py	90.90% <100.00%> (+0.90%)	⬆️
piccolo/query/methods/update.py	89.74% <0.00%> (-4.38%)	⬇️
piccolo/table.py	95.54% <0.00%> (ø)
piccolo/query/methods/delete.py	100.00% <0.00%> (ø)

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