StringSegment doesn't handle null buffers consistently

[GrabYourPitchforks]

Microsoft.Extensions.Primitives.StringSegment does not handle null backing buffers consistently. One example:

Console.WriteLine(default(StringSegment).IndexOfAny(new[] { 'x' })); // prints -1
Console.WriteLine(default(StringSegment).IndexOf('x')); // throws ArgumentOutOfRangeException
Console.WriteLine(default(StringSegment).LastIndexOf('x')); // prints -1

(Related: #39140)

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Issue Details

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Microsoft.Extensions.Primitives.StringSegment does not handle null backing buffers consistently. One example:

Console.WriteLine(default(StringSegment).IndexOfAny(new[] { 'x' })); // prints -1
Console.WriteLine(default(StringSegment).IndexOf('x')); // throws ArgumentOutOfRangeException

(Related: #39140)

Author:	GrabYourPitchforks
Assignees:	-
Labels:	area-Extensions-Primitives, bug
Milestone:	-