Fixing structured dataset for empty bq tables

[dylanwilder]

TL;DR

Structured dataset bq read fails on empty table because pandas.concat() fails if the passed array is empty. Fix it to create an empty dataframe with the required schema

Type

• [X] Bug Fix
• [ ] Feature
• [ ] Plugin

Are all requirements met?

• [ ] Code completed
• [ ] Smoke tested
• [ ] Unit tests added
• [ ] Code documentation added
• [ ] Any pending items have an associated Issue

Complete description

How did you fix the bug, make the feature etc. Link to any design docs etc

Tracking Issue

https://github.com/flyteorg/flyte/issues/

Follow-up issue

NA OR https://github.com/flyteorg/flyte/issues/

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[codecov[bot]]

Codecov Report

Merging #1157 (8ba3d61) into master (1c75bd1) will decrease coverage by 0.01%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1157      +/-   ##
==========================================
- Coverage   68.51%   68.50%   -0.02%     
==========================================
  Files         288      288              
  Lines       26085    26089       +4     
  Branches     2918     2919       +1     
==========================================
  Hits        17871    17871              
- Misses       7735     7739       +4     
  Partials      479      479              

Impacted Files	Coverage Δ
flytekit/types/structured/bigquery.py	0.00% <0.00%> (ø)

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