handle while with empty condition

src/atrule/mod.rs

@@ -317,6 +317,11 @@ impl AtRule {
                 let mut stmts = Vec::new();
                 devour_whitespace(toks);
                 let cond = read_until_open_curly_brace(toks);
+
+                if cond.is_empty() {
+                    return Err("Expected expression.".into());
+                }
+
                 toks.next();
                 let scope = &mut scope.clone();
                 let body = read_until_closing_curly_brace(toks);

tests/while.rs

@@ -23,3 +23,7 @@ test!(
     "@while false {\na {\n    color: $b;\n  }\n  $b: $b + 1;\n}",
     ""
 );
+error!(
+    while_empty_condition,
+    "@while {}", "Error: Expected expression."
+);