common/json-schema : fix GBNF grammar generation for empty object schemas (#28279)

common/json-schema-to-grammar.cpp

@@ -748,6 +748,10 @@ private:
             optional_props.push_back("*");
         }
 
+        if (required_props.empty() && optional_props.empty()) {
+            return "\"{\" space \"}\"";
+        }
+
         std::string rule = "\"{\" space ";
         for (size_t i = 0; i < required_props.size(); i++) {
             if (i > 0) {

examples/json_schema_to_grammar.py

@@ -734,6 +734,9 @@ class SchemaConverter:
             )
             optional_props.append("*")
 
+        if not required_props and not optional_props:
+            return '"{" space "}"'
+
         rule = '"{" space '
         rule += ' "," space '.join(prop_kv_rule_names[k] for k in required_props)
 

tests/test-json-schema-to-grammar.cpp

@@ -994,7 +994,7 @@ static void test_all(const std::string & lang, std::function<void(const TestCase
             "additionalProperties": false
         })""",
         R"""(
-            root ::= "{" space  space "}"
+            root ::= "{" space "}"
             space ::= | " " | "\n"{1,2} [ \t]{0,20}
         )"""
     });