Fix StringSegment.Buffer can be null

[maxkoshevoi]

Addresses: #57395 (comment)

• Made Buffer nullable
• Equals(text) returns true if both Buffer and text are null

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Issue Details

---

Addresses: #57395 (comment)

• Made Buffer nullable
• Return original implementation of Equals (throws on null)

Author:	maxkoshevoi
Assignees:	-
Labels:	new-api-needs-documentation, area-Extensions-Primitives
Milestone:	-

[drieseng]

Perhaps also add a test where null is passed to .ctor(String), and assert that:

• Buffer returns null
• Offset return zero
• Length returns zero
• HasValue returns false
• Value returns null
• ...

[maxkoshevoi]

There is one already: https://github.com/dotnet/runtime/blob/main/src/libraries/Microsoft.Extensions.Primitives/tests/StringSegmentTest.cs#L164
Nullable is not enabled for tests, so didn't notice it when doing #57395

Update: Added more asserts in it

[drieseng]

@maxkoshevoi Missed that one, thx.

[JamesNK]

FYI this PR is blocking the runtime -> aspnetcore merge: dotnet/aspnetcore#35547

If it is in a good state (@eerhardt or @stephentoub could you review the latest changes?) then it would be great to get it merged and unblock updates.

[stephentoub]

Thanks for fixing it.