BigQuery V2 Minimal API Cleanup : Remove is_null from the ColumnData struct

This cleanup is result of code review request for PR-14384 . More details [here](https://github.com/googleapis/google-cloud-cpp/pull/14384#pullrequestreview-2146018253).

Basically, an `is_null` parameter was added to[ `ColumnData` struct](https://github.com/googleapis/google-cloud-cpp/blob/8d01ae34c018c34a1002187705385775e70a4113/google/cloud/bigquery/v2/minimal/internal/common_v2_resources.h#L317) to distinguish between empty strings and null values.

The cleanup is to use `absl::optional` instead as mentioned in the code review feedback linked above.

Start with ColumnData in google/cloud/bigquery/v2/minimal/internal/common_v2_resources.h and read the linked PR-14384 review feedback for the intended cleanup. Trace the existing is_null uses and replace the null-state representation with absl::optional; done means empty strings remain distinguishable from null values without the separate parameter.

Vom Indexierungsmodell aus dem Issue-Text verfasst.