FromEnumString return unexpected result if input is invalid

FirstOrDefault return not null, but default value, so method never throw exeption:

public static T FromEnumString<T>(string value) where T : struct, Enum
{
    value.ThrowIfNull();

    var reflectionAttributeFinder = new ReflectionAttributeFinder();

    IReadOnlyDictionary<T, EnumStringValueAttribute> enumStrings = reflectionAttributeFinder.GetAttributesFromEnumValues<EnumStringValueAttribute, T>();

    KeyValuePair<T, EnumStringValueAttribute>? selectedEnumValue = enumStrings.FirstOrDefault(p => p.Value.StringValue == value);

    if (selectedEnumValue is null)
        throw new ReflectionException($"Cannot find enum values associated to string {value} in {typeof(T).Name}");

    return selectedEnumValue.Value.Key;
}

Start at the FromEnumString entry point shown in the issue and inspect how FirstOrDefault is checked when no enum value matches. Add coverage for an invalid string and verify that it throws ReflectionException, while existing valid-value behavior remains unchanged.