Clear generated custom cell part names while rows are loading

[rolfsmeds]

Description

Due to the way Grid reuses rows, custom generated cell part names are incorrectly applied to rows while they are loading. See workaround applied in docs sample: vaadin/docs#2866.

Expected outcome

Rows that are still loading should not have any custom part names applied to their cells.

Minimal reproducible example

https://vaadin.com/docs/latest/components/grid#part-name-generator

Steps to reproduce

1. Apply part names to cells based on their contents with PartNameGenerator
2. Scroll the Grid
3. Observe the part name based styles incorrectly applied to rows that are still loading

Environment

Vaadin version(s): V24

Browsers

Issue is not browser related