Ignore headers with null values in web cmdlets

[iSazonov]

PR Summary

Core throws if web header has a null value - it is not valid value.
Before the change web cmdlets thrown with NRE (value.ToString()).
Now we silently ignore such headers.

PR Context

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[SteveL-MSFT]

Should we also fix the issue with empty string in this PR?

[iSazonov]

@SteveL-MSFT It is another code path. I think we should have different commits.

[ghost]

🎉v7.1.0-preview.1 has been released which incorporates this pull request.:tada:

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