Support overriding virtual indexer property in proxies

[ajcvickers]

Fixes #21985

[ajcvickers]

@Pilchie RC1

[smitpatel]

May be should also remove line from #21948 (comment)

[ajcvickers]

@smitpatel Dictionary is not proxyable. We might want to throw if people try to use it as a proxy type, but that would also fail all our current tests. We should discuss in the context of #22009.

[Pilchie]

Approved for RC1.

[ajcvickers]

@smitpatel @AndriySvyryd Added another commit which improves this significantly by handling null values returned from indexer properties in the same way we handle null values in the backing field of a non-nullable property.