IOP: Fix FetchOperationLimits for some use cases

[mregen]

Proposed changes

• FetchOperationLimits may throw null pointer exception if server MaxByteStringLength is empty or null.

In this case operation limits were ignored and other BadTooManyOperations errors may be returned because default operation limits were used.

Related Issues

• Fixes #

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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• [ ] I have read the CONTRIBUTING doc.
• [ ] 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 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 54.82%. Comparing base (3672ebe) to head (ea5bd15). Report is 2 commits behind head on develop/main374.

Files with missing lines	Patch %	Lines
Libraries/Opc.Ua.Client/Session/Session.cs	66.66%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop/main374 #2807 +/- ## =================================================== - Coverage 57.23% 54.82% -2.41% =================================================== Files 352 349 -3 Lines 75520 66056 -9464 Branches 15988 13574 -2414 =================================================== - Hits 43223 36216 -7007 + Misses 27870 25875 -1995 + Partials 4427 3965 -462 ```

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