RFR: Fix erasure of null-restricted types

[Vicente Romero]

On Thu, 8 Jan 2026 16:29:58 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> There are two issues with erasing null-restricted types:
> * not all the code paths in `Types::erasure` drop null restrictions
> * for some types like `String`, `Types::erasure` treats erasure as a no-op

lgtm

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Marked as reviewed by vromero (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/1870#pullrequestreview-3640260698