Fix wiki diff view with no line numbers

Fixes #626

src/plainui/utils.py

@@ -85,7 +85,7 @@ class StaticPageDiff(HtmlDiff):
         try:
             linenum = f'{linenum:d}'
             change_id = f' id="{self._prefix[side]}{linenum}"'
-        except TypeError:
+        except ValueError:
             # handle blank lines where linenum is '>' or ''
             change_id = ''