[release/5.0] ignore empty domain for digest auth

[github-actions]

Backport of #50348 to release/5.0

Fixes #50283

/cc @wfurt

Customer Impact

This breaks Atlas MongoDB. As popular framework it has impact on wider user group.
While technically this is not regression in SocketHttpHandler as it never work before, option to use legacy handler was removed in 5.0 so there is no good solution.

The fix is small as our validation was too strict as RFC allows empty token but we failed to parse and reported as invalid.

Testing

Reported verified fix with daily 6.0 build. New test case was also added.

Risk

Small

[ghost]

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Issue Details

---

Backport of #50348 to release/5.0

/cc @wfurt

Customer Impact

Testing

Risk

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.Net.Http
Milestone:	-

[danmoseley]

@wfurt tactics asked to please ensure it's verified this fixes MongoDB (using 6.0 bits is OK). It says it was verified with 6.0 but not clear whether that's MongoDB.

[wfurt]

Yes, it was verified by the reporter to so I assume it was MongoDB. cc: @gao-artur to clarify.

[gao-artur]

Yes, verified it with Atlas MongoDB and it worked.

[danmoseley]

Ok great then @Anipik can merge when green.

[Anipik]

+ export __CrashDumpFolder=/home/helixbot/dotnetbuild/dumps
+ cat /root/helix/work/workitem/SetStressModes_normal.sh
#!/usr/bin/env bash
export COMPlus_DbgEnableMiniDump=1
export COMPlus_DbgMiniDumpName=$HELIX_DUMP_FOLDER/coredump.%d.dmp
+ export PATH=/root/helix/work/correlation/dotnet-cli:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+ export DOTNET_ROOT=/root/helix/work/correlation/dotnet-cli
+ export DOTNET_CLI_TELEMETRY_OPTOUT=1
+ dotnet /root/helix/work/correlation/xunit.console.dll JIT/Generics/JIT.Generics.XUnitWrapper.dll -parallel collections -nocolor -noshadow -xml testResults.xml -trait TestGroup=JIT.Generics
Microsoft.DotNet.XUnitConsoleRunner v2.5.0 (64-bit .NET 5.0.0)
  Discovering: JIT.Generics.XUnitWrapper
  Discovered:  JIT.Generics.XUnitWrapper
  Starting:    JIT.Generics.XUnitWrapper
  Finished:    JIT.Generics.XUnitWrapper
=== TEST EXECUTION SUMMARY ===
   JIT.Generics.XUnitWrapper  Total: 41, Errors: 0, Failed: 0, Skipped: 0, Time: 4.830s
+ export _commandExitCode=0

the leg timed out after passing all the tests., hence going forward and merging this one.