Error thrown when UserAgent is null

[angelofan]

From RFC7231 User-Agent :

A user agent SHOULD send a User-Agent field in each request unless specifically configured not to do so.

From Microsoft Dotnet Api :

The UserAgent is null when there is no User-Agent header in the request.

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The header is a web standard, but we can not guarantee that is present. For example if the call is made with cURL, HttpClient, low level component, ...

The code must be "null-safe", it should not generate an exception if UserAgent is null.

[angelofan]

Although it is possible to avoid passing null in with some pre-judgment, it is easy to forget. For example, every time I use this package for a new project, this problem will occur again when I forget to write some pre-judgment.