An enum to indicate how to interpret missing values. Missing
values are fields present in user schema but missing in rows. A
missing value can represent a NULL or a column default value
defined in BigQuery table schema.
Values:
MISSING_VALUE_INTERPRETATION_UNSPECIFIED (0):
Invalid missing value interpretation.
Requests with this value will be rejected.
NULL_VALUE (1):
Missing value is interpreted as NULL.
DEFAULT_VALUE (2):
Missing value is interpreted as column
default value if declared in the table schema,
NULL otherwise.