SQL Expressions: (Chore) Update code for latest plugin-sdk data pkg (#100425)

Use new NilAt and SetRefId methods

pkg/expr/sql/frame_db_conv.go

@@ -410,65 +410,3 @@ func fieldValFromRowVal(fieldType data.FieldType, val interface{}) (interface{},
 		return nil, fmt.Errorf("unsupported field type %s for val %v", fieldType, val)
 	}
 }
-
-// Is the field nilAt the index. Can panic if out of range.
-// TODO: Maybe this should be a method on data.Field?
-func nilAt(field data.Field, at int) bool {
-	if !field.Nullable() {
-		return false
-	}
-
-	switch field.Type() {
-	case data.FieldTypeNullableInt8:
-		v := field.At(at).(*int8)
-		return v == nil
-
-	case data.FieldTypeNullableUint8:
-		v := field.At(at).(*uint8)
-		return v == nil
-
-	case data.FieldTypeNullableInt16:
-		v := field.At(at).(*int16)
-		return v == nil
-
-	case data.FieldTypeNullableUint16:
-		v := field.At(at).(*uint16)
-		return v == nil
-
-	case data.FieldTypeNullableInt32:
-		v := field.At(at).(*int32)
-		return v == nil
-
-	case data.FieldTypeNullableUint32:
-		v := field.At(at).(*uint32)
-		return v == nil
-
-	case data.FieldTypeNullableInt64:
-		v := field.At(at).(*int64)
-		return v == nil
-
-	case data.FieldTypeNullableUint64:
-		v := field.At(at).(*uint64)
-		return v == nil
-
-	case data.FieldTypeNullableFloat64:
-		v := field.At(at).(*float64)
-		return v == nil
-
-	case data.FieldTypeNullableString:
-		v := field.At(at).(*string)
-		return v == nil
-
-	case data.FieldTypeNullableTime:
-		v := field.At(at).(*time.Time)
-		return v == nil
-
-	case data.FieldTypeNullableBool:
-		v := field.At(at).(*bool)
-		return v == nil
-
-	default:
-		// Either it's not a nullable type or it's unsupported
-		return false
-	}
-}