Check if enum_values exists in the serverUrl [csharp]

[KaiUweHueber-TRUMPF]

If no enum_values are given for in a serverUrl, the check should be ignored. Currently the serverVariable must contain "enum_values" otherwise the function throws an exception that serverVariables does not contain "enum_values"

@mandrean (2017/08) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05) @iBicha (2023/07)

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[wing328]

appveyor and other CI tests passed via #21127