patch 8.2.1300: Vim9: optional argument type not parsed properly

Problem:    Vim9: optional argument type not parsed properly.
Solution:   Skip over the "?". (issue #6507)
diff --git a/src/evalvars.c b/src/evalvars.c
index e11fdac..f409f8c 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1013,7 +1013,7 @@
 	if (end == arg + 2 && end[-1] == ':')
 	    --end;
 	if (*end == ':')
-	    end = skip_type(skipwhite(end + 1));
+	    end = skip_type(skipwhite(end + 1), FALSE);
     }
     return end;
 }