Update Value docs re: zero-value behavior (#60)

ulid.go

@@ -469,19 +469,38 @@ func (id *ULID) Scan(src interface{}) error {
 	return ErrScanValue
 }
 
-// Value implements the sql/driver.Valuer interface. This returns the value
-// represented as a byte slice. If instead a string is desirable, a wrapper
-// type can be created that calls String().
+// Value implements the sql/driver.Valuer interface, returning the ULID as a
+// slice of bytes, by invoking MarshalBinary. If your use case requires a string
+// representation instead, you can create a wrapper type that calls String()
+// instead.
 //
-//	// stringValuer wraps a ULID as a string-based driver.Valuer.
-// 	type stringValuer ULID
+//    type stringValuer ulid.ULID
 //
-//	func (id stringValuer) Value() (driver.Value, error) {
-//		return ULID(id).String(), nil
-//	}
+//    func (v stringValuer) Value() (driver.Value, error) {
+//        return ulid.ULID(v).String(), nil
+//    }
+//
+//    // Example usage.
+//    db.Exec("...", stringValuer(id))
+//
+// All valid ULIDs, including zero-value ULIDs, return a valid Value with a nil
+// error. If your use case requires zero-value ULIDs to return a non-nil error,
+// you can create a wrapper type that special-cases this behavior.
+//
+//    var zeroValueULID ulid.ULID
+//
+//    type invalidZeroValuer ulid.ULID
+//
+//    func (v invalidZeroValuer) Value() (driver.Value, error) {
+//        if ulid.ULID(v).Compare(zeroValueULID) == 0 {
+//            return nil, fmt.Errorf("zero value")
+//        }
+//        return ulid.ULID(v).Value()
+//    }
+//
+//    // Example usage.
+//    db.Exec("...", invalidZeroValuer(id))
 //
-//	// Example usage.
-//	db.Exec("...", stringValuer(id))
 func (id ULID) Value() (driver.Value, error) {
 	return id.MarshalBinary()
 }