[api/v1][query_obj] add default prequeries array

[williaster]

This PR only affects the embeddable charts /api/v1/query endpoint. I'm working on adding react <ChartDataProvider /> components in @superset-ui and when I use the new (non-legacy) WordCloudPlugin together with SuperChart + ChartClient I get the following stack trace from Python in the Superset App:

POST /api/v1/query/ => {"datasource":{"id":3,"type":"table"},"queries":[{"granularity":"ds","metrics":[{"label":"sum__num"}],"groupby":["name"]}]}

2019-03-14 17:24:21,854:ERROR:root:'NoneType' object has no attribute 'append'
Traceback (most recent call last):
  File "/Users/chris_williams/dev/superset/superset/common/query_context.py", line 206, in get_df_payload
    query_result = self.get_query_result(query_obj)
  File "/Users/chris_williams/dev/superset/superset/common/query_context.py", line 82, in get_query_result
    result = self.datasource.query(query_object.to_dict())
  File "/Users/chris_williams/dev/superset/superset/connectors/sqla/models.py", line 861, in query
    query_obj['prequeries'].append(sql)

This is because prequeries is not passed for this chart, and there is no default on the Python side. This PR adds a default and updates the type, I think it's an optional List of Dicts, setting the default to {} errored downstream as something was trying to append to it 😄 so maybe it was typed wrong?

@xtinec @conglei @kristw

[conglei]

Thanks for fixing this!

[codecov-io]

Codecov Report

Merging #7039 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #7039   +/-   ##
=======================================
  Coverage   64.45%   64.45%           
=======================================
  Files         421      421           
  Lines       20524    20524           
  Branches     2245     2245           
=======================================
  Hits        13229    13229           
  Misses       7168     7168           
  Partials      127      127

Impacted Files	Coverage Δ
superset/common/query_object.py	26.31% <0%> (ø)	⬆️

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