Client ReadNodes, throw BadInvalidType if a value type returned by an attribute is null

[mregen]

Proposed changes

Handle a case where a misconfigured server returns a null value for an attribute when a value type is expected. To avoid the NullReferenceException that stops the whole ReadNodes operation, flag the Node with the service result and or return a default value if required.

Related Issues

More comprehensive fix for

• Fixes #2729,
• Fixes #2638

Types of changes

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• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] Enhancement (non-breaking change which adds functionality)
• [ ] Test enhancement (non-breaking change to increase test coverage)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

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• [x] I have read the CONTRIBUTING doc.
• [x] I have signed the CLA.
• [ ] I ran tests locally with my changes, all passed.
• [ ] I fixed all failing tests in the CI pipelines.
• [ ] I fixed all introduced issues with CodeQL and LGTM.
• [ ] I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• [ ] I have added necessary documentation (if appropriate).
• [ ] Any dependent changes have been merged and published in downstream modules.

Further comments

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 62.50000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 54.73%. Comparing base (4bbb062) to head (2613b77). Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
Stack/Opc.Ua.Core/Types/BuiltIn/DataValue.cs	16.66%	6 Missing and 4 partials :warning:
Libraries/Opc.Ua.Client/Session/Session.cs	90.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2746 +/- ## ========================================== + Coverage 54.71% 54.73% +0.02% ========================================== Files 349 349 Lines 65683 65695 +12 Branches 13438 13443 +5 ========================================== + Hits 35938 35958 +20 + Misses 25853 25844 -9 - Partials 3892 3893 +1 ```

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