Don't throw in CredentialSettings ctor for non-auth clients

[Copilot]

Description

The CredentialSettings constructor threw ArgumentNullException when passed a null section, preventing non-auth clients from initializing without credential configuration.

Changes:

• Modified CredentialSettings constructor to return early when section is null instead of throwing
• Added test coverage for null section handling

Before:

public CredentialSettings(IConfigurationSection section)
{
    if (section is null)
    {
        throw new ArgumentNullException(nameof(section));
    }
    // ...
}

After:

public CredentialSettings(IConfigurationSection section)
{
    if (section is null)
    {
        return;  // Properties remain null, suitable for non-auth clients
    }
    // ...
}

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• I have read the contribution guidelines.
• The pull request does not introduce breaking changes.

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• Title of the pull request is clear and informative.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Don't throw in credential settings ctor for non auth clients</issue_title>
<issue_description>https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/core/System.ClientModel/src/Convenience/CredentialSettings.cs#L23

Just return when the credential section is null</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Don't throw in credential settings ctor for non auth clients #56242

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[Copilot]

Pull request overview

Adjusts System.ClientModel convenience authentication configuration to allow clients that don’t require auth to initialize even when no credential configuration section is provided.

Changes:

• Updated CredentialSettings(IConfigurationSection section) to return early when section is null (instead of throwing).
• Added a unit test covering the null-section constructor scenario.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
sdk/core/System.ClientModel/src/Convenience/CredentialSettings.cs	Allows CredentialSettings construction with a null configuration section by returning early.
sdk/core/System.ClientModel/tests/Convenience/CredentialSettingsTests.cs	Adds coverage asserting the ctor does not throw and leaves properties null/default when passed null.

[m-nash]

/check-enforcer override