[hotfix] Stabilize integration test (derby)

Author: och5351

flink-connector-jdbc-core/src/main/java/org/apache/flink/connector/jdbc/core/datastream/source/reader/JdbcSourceSplitReader.java

@@ -146,9 +146,15 @@ public RecordsWithSplitIds<RecordAndOffset<T>> fetch() throws IOException {
                 // Check if ResultSet is still open before calling next()
                 // This is needed for databases like Derby where ResultSet might be closed
                 // when autocommit is disabled
-                if (!resultSet.isClosed()) {
-                    hasNextRecordCurrentSplit = resultSet.next();
-                } else {
+                try {
+                    if (!resultSet.isClosed()) {
+                        hasNextRecordCurrentSplit = resultSet.next();
+                    } else {
+                        hasNextRecordCurrentSplit = false;
+                    }
+                } catch (SQLException e) {
+                    // Connection might be closed (e.g., PostgreSQL throws exception on
+                    // isClosed() if connection is closed)
                     hasNextRecordCurrentSplit = false;
                 }
             } catch (Exception e) {