Spanner now supports the SAFE_TO_JSON function in GoogleSQL-dialect databases

## Feature Spanner now supports the [SAFE\_TO\_JSON](https://cloud.google.com/spanner/docs/reference/standard-sql/json%5Ffunctions#safe%5Fto%5Fjson) function in GoogleSQL-dialect databases. You can use this function to convert SQL objects to JSON objects. Unlike [TO\_JSON](https://cloud.google.com/spanner/docs/reference/standard-sql/json%5Ffunctions#to%5Fjson), this function converts invalid JSON types to JSON null values, rather than errors.