Queries with nested fields for presto fail with " 'The number of data values and number of nestedfields are not equal'"

A clear and concise description of what the bug is. When querying a deeply nested field in presto, a “The number of data values and number of nestedfields are not equal’” error. This appears related to http://mail-archives.apache.org/mod_mbox/superset-notifications/201905.mbox/<[e-mail]>

Expected results

The data

Actual results

Exception: ‘The number of data values and number of nestedfields are not equal’

Screenshots

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How to reproduce the bug

1. Query a deeply nested field (a row of array of row) in sql lab
2. If the rows have different shape, i.e. one is null, the other has data only the shape of the first row determines the data.
3. The code comes from ./superset/db_engine_specs/presto.py in the _expand_row_data function.

Environment

(please complete the following information):

• superset version: 0.34.1rc1
• python version: python --version 3.6
• node.js version: node -v
• npm version: npm -v

Checklist

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• [X ] I have checked the superset logs for python stacktraces and included it here as text if there are any.
• [ X] I have reproduced the issue with at least the latest released version of superset.
• [ X] I have checked the issue tracker for the same issue and I haven’t found one similar.

Additional context

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[willbarrett]

Hi @elonazoulay - feel free to contribute that fix back to core. Thanks for digging in!

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