readme: bigquery: correct example

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Fixes #<issue_number_goes_here> 🦕

🤖 I have created a release \*beep\* \*boop\*
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## [2.21.0](https://www.github.com/googleapis/gax-nodejs/compare/v2.20.0...v2.21.0) (2021-08-02)


### Features

* decode error details property from google.rpc.Status ([#1057](https://www.github.com/googleapis/gax-nodejs/issues/1057)) ([2dde63e](https://www.github.com/googleapis/gax-nodejs/commit/2dde63e87c11144e79e523df48756c60c119d404))


### Bug Fixes

* clean up the lint ([#1067](https://www.github.com/googleapis/gax-nodejs/issues/1067)) ([d08e9d5](https://www.github.com/googleapis/gax-nodejs/commit/d08e9d5d67df89a4cecc73f8c00a76adc5e393a2))
---


This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please).

* fix: Allows nulls to be passed into value

A user expressed interest in filtering with a null value. This typescript parameter change will now allow users to do so.

fixes #958

* Add tests in the system test folder

The code in the system test folder reflects the user experience more closely so we want to provide test cases there that will break when compiling typescript if new changes in src are not provided.

* Assert statements in system tests

Add assert statements to check that the value in the filter created in the system test actually equals null.

* Modify tests to assert check for null

Slight rename in system tests. Modified tests so that they test to see that the filter has a null value.

* Prefer elvis operator in assert statements

This change replaces the a longer fragment with elvis operator to make assert statements more concise.

* eliminate the need for a variable

In two tests we eliminate variables and just have an assert statement so that we can have more concise code.

* fix: Allows nulls to be passed into value

A user expressed interest in filtering with a null value. This typescript parameter change will now allow users to do so.

fixes #958

* Add tests in the system test folder

The code in the system test folder reflects the user experience more closely so we want to provide test cases there that will break when compiling typescript if new changes in src are not provided.

* Assert statements in system tests

Add assert statements to check that the value in the filter created in the system test actually equals null.

* Modify tests to assert check for null

Slight rename in system tests. Modified tests so that they test to see that the filter has a null value.

* Prefer elvis operator in assert statements

This change replaces the a longer fragment with elvis operator to make assert statements more concise.

* eliminate the need for a variable

In two tests we eliminate variables and just have an assert statement so that we can have more concise code.