Relax the rules for what counts as a well formed GUID.

[DDEfromOR]

The previous regex pattern used to determine if a header's identifier GUID was well formed enforced the timestamp based UUID rules as laid out in RFC 4122. The dotnet standard library used in C# bots only requires a GUID to match the 8-4-4-4-12 pattern to be valid, making them less restrictive.

This change relaxes the regex matching to match any GUID following the 8-4-4-4-12 pattern for parity with the C# library.

[coveralls]

Pull Request Test Coverage Report for Build 83255

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.976%

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Totals
Change from base Build 83243:	0.0%
Covered Lines:	4743
Relevant Lines:	5241

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💛 - Coveralls

[carlosscastro]

would love to see a test for the bug that made us find this, or at least specific tests

[DDEfromOR]

Added a test to make sure nil GUIDs are accepted.