ECMA-335 addendum should mention that null managed pointers are legal

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Ref: https://github.com/dotnet/runtime/blob/main/docs/design/specs/Ecma-335-Augments.md

We expose APIs like Unsafe.IsNullRef and Unsafe.NullRef, which imply that nullptr managed pointers are supported across our runtimes. However, ECMA-335, Sec. II.14.4.2 and III.1.1.5.2 both explicitly state that nullptr managed pointers are disallowed. ECMA should be updated.

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Additional note: we might want to point out that managed pointers just past the end of managed objects are legal. ECMA today only says managed pointers just past the end of managed arrays are legal. As fixed buffers take off, we might have more occurrences of people treating non-array objects as array-like.