Disallow empty dimension

Author: gvanrossum

Tools/cases_generator/parser.py

@@ -261,6 +261,8 @@ def dimension(self) -> Dimension | None:
         while (tkn := self.peek()) and tkn.kind != lx.RBRACKET:
             tokens.append(tkn)
             self.next()
+        if not tokens:
+            return None
         return Dimension(lx.to_text(tokens).strip())
 
     @contextual