Describe the purpose of your pull request

The SDK has a long-standing bug that occurs when the client instance is used in a hook event handler during client initialization. For example, a setup like

ConfigCatClient.Get("SDK-KEY", options =>
{
    options.ConfigChanged += (s, e) => snapshot = ((IConfigCatClient)s!).GetValue(...);
});      

blows up when up-to-date config data is already available in the cache because in that case the event is raised before all fields are initialized in ConfigCatClient . A call like GetValue accesses those fields, resulting in a NullReferenceException exception.

This PR fixes this issue.

Related issues (only if applicable)

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How to test? (only if applicable)

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Security (only if applicable)

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Requirement checklist (only if applicable)

• I have covered the applied changes with automated tests.
• I have executed the full automated test set against my changes.
• I have validated my changes against all supported platform versions.
• I have read and accepted the contribution agreement.