`@font-face` `font-weight` set to a range becomes empty after parsing

[albi005]

Prerequisites

• [x] Can you reproduce the problem in a MWE?
• [x] Are you running the latest version of AngleSharp.Css?
• [x] Did you check the FAQs to see if that helps you?
• [x] Are you reporting to the correct repository? (there are multiple AngleSharp libraries, e.g., AngleSharp.Xml for Xml support)
• [x] Did you perform a search in the issues?

Steps to Reproduce

string css = """
    @font-face {
      font-weight: 100 700;
    }
""";
string output = new CssParser()
    .ParseStyleSheet(css)
    .ToCss();

Expected Behavior

output contains font-weight: 100 700;

Actual Behavior

output contains font-weight: ;