An enum to indicate how to interpret missing values of fields
that are 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.
Enum 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.