test(bigquery): fix array concat test

[cpcloud]

Fixes the upstream failing test cases for bigquery.

[cpcloud]

Tests are now correctly xfailing:

ibis/backends/tests/test_array.py::test_array_concat_with_null[bigquery-right] XFAIL (bigquery treats null arrays as empty and a ValueError is raised from `pd.isna` when the value is not a
truthy value)                                                            [ 50%]
ibis/backends/tests/test_array.py::test_array_concat_with_null[bigquery-left] XFAIL (bigquery treats null arrays as empty and a ValueError is raised from `pd.isna` when the value is not a
truthy value)                                                            [100%]

================================================================================= 36 deselected, 2 xfailed in 50.23s =================================================================================