ReadClient throws on null values

[kitsunde]

https://github.com/googleapis/nodejs-bigquery-storage/blob/main/src/reader/arrow_transform.ts#L146 this can be null and null is an object, which causes nullable fields to throw. For example in this case:

fieldData.toString();
>>> {"column_name": null}

Then it gets passed to https://github.com/googleapis/nodejs-bigquery-storage/blob/main/src/reader/arrow_transform.ts#L159 and enter the object path because typeof null === 'object' is true then break on Object.keys(null) in https://github.com/googleapis/nodejs-bigquery-storage/blob/main/src/reader/arrow_transform.ts#L168

TypeError: Cannot convert undefined or null to object

[alvarowolfx]

thanks for the report @kitsunde, I'll work on fixing that and improving the test coverage for that