yomidbbuilder: support null values for the def tags

A term bank containing a term with its def tags set to null would cause
an error because only strings were allowed even though the yomichan
dictionary term bank schema allows not only strings but also nulls
(treated the same as empty strings "").

src/dict/yomidbbuilder.c

@@ -996,9 +996,26 @@ static int add_term(sqlite3 *db, json_object *term, const sqlite3_int64 id)
         goto cleanup;
     reading = json_object_get_string(ret_obj);
 
-    if ((ret = get_obj_from_array(term, DEF_TAGS_INDEX, json_type_string, &ret_obj)))
-        goto cleanup;
-    def_tags = json_object_get_string(ret_obj);
+    ret_obj = json_object_array_get_idx(term, DEF_TAGS_INDEX);
+    switch (json_object_get_type(ret_obj))
+    {
+        case json_type_string:
+            def_tags = json_object_get_string(ret_obj);
+            ret = 0;
+            break;
+        case json_type_null:
+            def_tags = "";
+            ret = 0;
+            break;
+        default:
+            fprintf(stderr, "Expected index %u to be of type string or null\n%s\n",
+                    DEF_TAGS_INDEX,
+                    json_object_to_json_string_ext(term, JSON_C_TO_STRING_PRETTY
+            ));
+            ret_obj = NULL;
+            ret = JSON_WRONG_TYPE_ERR;
+            goto cleanup;
+    }
 
     if ((ret = get_obj_from_array(term, RULES_INDEX, json_type_string, &ret_obj)))
         goto cleanup;