need to allow empty strings when a string property has a format

src/tests.rs

@@ -1237,3 +1237,53 @@ fn test_unevaluated_properties_errors() {
   let valid_result = validate_json_schema(schema_id, jsonb(valid_instance));
   assert_success_with_json!(valid_result, "All properties are evaluated, should pass");
 }
+
+#[pg_test]
+fn test_format_validation_allows_empty_string() {
+  clear_json_schemas();
+  let schema_id = "format_schema_empty";
+  let schema = json!({
+    "type": "object",
+    "properties": {
+      "uuid": { "type": "string", "format": "uuid" },
+      "date_time": { "type": "string", "format": "date-time" },
+      "email": { "type": "string", "format": "email" }
+    }
+  });
+  
+  let _ = cache_json_schema(schema_id, jsonb(schema), false);
+  
+  // Test with empty strings for all formatted fields
+  let instance = json!({
+    "uuid": "",
+    "date_time": "",
+    "email": ""
+  });
+  
+  let result = validate_json_schema(schema_id, jsonb(instance));
+  
+  // This is the test that should fail before the change and pass after
+  assert_success_with_json!(result, "Empty strings should be allowed for format validation");
+}
+
+#[pg_test]
+fn test_non_empty_string_format_validation_still_fails() {
+  clear_json_schemas();
+  let schema_id = "non_empty_fail_schema";
+  let schema = json!({
+    "type": "object",
+    "properties": {
+      "date_time": { "type": "string", "format": "date-time" }
+    }
+  });
+  
+  let _ = cache_json_schema(schema_id, jsonb(schema), false);
+  
+  // A non-empty but invalid string should still fail
+  let instance = json!({
+    "date_time": "not-a-date"
+  });
+  
+  let result = validate_json_schema(schema_id, jsonb(instance));
+  assert_failure_with_json!(result, 1, "Value \"not-a-date\" is not a valid date-time format");
+}