Support empty array in _from_api_repr_struct

[TommyDew42]

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Fixes #2009 🦕

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[chalmerlowe]

@TommyDew42 Thank you for suggesting a code improvement. It does appear that we may need to make our code logic more robust in that area.

Right now, this PR does not appear to have a matching test.
Issue #2009 refers to the case where the ArrayQueryParameter is empty.

The ArrayQueryParameter class must take a StructQueryParameterType if the values is an empty array (docstring):

If the type is 'STRUCT'/'RECORD' and values is empty,
the exact item type cannot be deduced, thus a StructQueryParameterType
instance needs to be passed in.

A test similar to test_from_api_repr_w_struct_type, but with an empty values array, may be suitable.

With one or more test cases to exercise the logic, we will also need to ensure that coverage passes:
https://github.com/googleapis/python-bigquery/blob/main/CONTRIBUTING.rst

If you are not sure how to proceed with producing tests, let us know, we might be able to offer some guidance.