Allow string-valued parameters to XSPICE device models with no default.

The code model then sees the value NULL.  Needed for the "family"
parameter on logic models, used by automatic bridge insertion.

src/xspice/mif/mif_inp2.c

@@ -568,8 +568,15 @@ MIF_INP2A (
         param_info = &(DEVices[type]->DEVpublic.param[i]);
 
         if(mdfast->param[i]->is_null) {
+            char* emessage;
+
             if(! param_info->has_default) {
-                LITERR("Parameter on model has no default");
+                if (param_info->type == MIF_STRING)
+                    continue;   // Allow NULL
+                emessage = tprintf("Parameter %s on model %s has no default",
+                                   param_info->name, mdfast->gen.GENmodName);
+                LITERR(emessage);
+                tfree(emessage);
                 gc_end();
                 return;
             } else if((param_info->is_array) && (! param_info->has_conn_ref)) {