Exploded @query array/dictionary parameters backed by a string struct (extensible string enum) generated code that passed .ToString() directly to AppendQuery(..., escape: true). A default-constructed value has a null internal string, so .ToString() returns null and Uri.EscapeDataString throws ArgumentNullException at runtime.

Changes

• Null/empty guard on exploded elements (RestClientProvider.BuildAppendQueryStatement): wraps each AppendQuery in if (!string.IsNullOrEmpty(...)) for array and dictionary explode when the element/value is a string-backed extensible enum. Gated by a new IsExtensibleStringEnum helper (IsEnum && IsStruct && UnderlyingEnumType == typeof(string)) so int/float/double enums and plain values are unaffected.
• Dictionary explode value fix: the enum was serialized from the KeyValuePair (item) rather than item.Value, yielding wrong output (e.g. @param.ToSerialString(), ((int)@param)). Now uses item.Value, correcting the p6/p7 paths as well.
• Tests: extended TestBuildCreateRequestMethodWithQueryParameters with extensible string enum array + dictionary parameters; baseline updated.

Generated output now:

foreach (var @param in include)
{
    if (!string.IsNullOrEmpty(@param.ToString()))
    {
        uri.AppendQuery("include[]", @param.ToString(), true);
    }
}

• Fixes [Bug]: C# emitter does not guard against null ToString() when serializing exploded query parameters backed by string structs #11431