List upcast converters should not remove invalid nodes.

[niegowski]

Suggested merge commit message (convention)

Fix (list): Text nodes directly inside <ul> or <ol> should not be removed while loading editor data. Closes #16450.

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Additional information

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[Witoso]

Lost items AFAICS wrap in p, so we can quickly avoid data loss situation, and just have data change. @Reinmar do you have any comments/historical context. IMHO works well enough to merge.

[Reinmar]

This change makes the content being preserved, but those incorrect pieces of HTML will split the surrounding lists (CKE4 would "auto-li" them).

Still, it's a simple change now while aligning perfectly to CKE4 might be much harder, so let's go for simplicity.