The SDF timing spec list can have zero or more elements.

The SDF file format supports zero or more timing specs. Previously
the parser was one or more. Things like tie high/low cells do not
have any delay information so zero or more is needed and matches
the standard.

vpi/sdf_parse.y

@@ -188,7 +188,7 @@ cell_list
 cell
   : '(' K_CELL celltype cell_instance
       { sdf_select_instance($3, $4); /* find the instance in the design */}
-    timing_spec_list
+    timing_spec_list_opt
     ')'
       { free($3);
 	if ($4) free($4);
@@ -216,9 +216,9 @@ cell_instance
 	    $$ = strdup(""); }
   ;
 
-timing_spec_list
-  : timing_spec_list timing_spec
-  | timing_spec
+timing_spec_list_opt
+  : /* Empty */
+  | timing_spec_list_opt timing_spec
   ;
 
 timing_spec