Issue #91 Fix additionalProperties default value in JTD validator

- Fixed JTD validator to correctly default additionalProperties to false when no properties are defined
- Added test case testAdditionalPropertiesDefaultsToFalse() to verify the fix
- Updated CI test count from 463 to 464 to account for new test
- This ensures RFC 8927 compliance where empty properties schemas reject additional properties by default

The bug was in Jtd.java line 446 where additionalProperties was set to true instead of false
when both properties and optionalProperties were empty. This caused empty schemas to incorrectly
allow additional properties instead of rejecting them by default.

Closes #91

Author: simbo1905

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
                       for k in totals: totals[k]+=int(r.get(k,'0'))
                   except Exception:
                       pass
-          exp_tests=463
+          exp_tests=464
           exp_skipped=0
           if totals['tests']!=exp_tests or totals['skipped']!=exp_skipped:
               print(f"Unexpected test totals: {totals} != expected tests={exp_tests}, skipped={exp_skipped}")

json-java21-jtd/src/main/java/json/java21/jtd/Jtd.java

@@ -442,8 +442,8 @@ JtdSchema compilePropertiesSchema(JsonObject obj) {
       }
       additionalProperties = bool.value();
     } else if (properties.isEmpty() && optionalProperties.isEmpty()) {
-      // Empty schema with no properties defined allows additional properties by default
-      additionalProperties = true;
+      // Empty schema with no properties defined rejects additional properties by default
+      additionalProperties = false;
     }
 
     return new JtdSchema.PropertiesSchema(properties, optionalProperties, additionalProperties);