NOT NULL on type other than ARRAY is not respected by fdb-relational

[g31pranjal]

While creating a table or struct, declaring a column type as NOT NULL is not honoured by the system since the nullability information gets lost in converting Relational DataType -> Record Layer Type -> Record Metadata.

ARRAY type is an exception to this as the nullability of Array is strictly defined by means of the NullableArrayWrapper - which is preserved in the RecordMetadata and its existence is indicative of Array's nullability.

Long Term: We need a solution to specify nullability.
Short Term: We should explicitly not allow defining a table with a NOT NULL non-array column.