fix: mock BackendUtil.getInstance in DorisSinkTest to prevent NPE

mockStatic intercepts all static methods but getInstance was not stubbed,
returning null. The batch mode constructor immediately calls
getAvailableBackend() on the null instance, causing an NPE.

Author: gnehil

flink-doris-connector/flink-doris-connector-flink1/src/test/java/org/apache/doris/flink/sink/DorisSinkTest.java

@@ -41,6 +41,7 @@
 import org.apache.flink.metrics.groups.SinkWriterMetricGroup;
 
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.mockStatic;
 import static org.mockito.Mockito.when;
@@ -51,8 +52,15 @@ public class DorisSinkTest {
 
     @Before
     public void setUp() throws Exception {
+        BackendUtil mockBackendUtil = mock(BackendUtil.class);
+        when(mockBackendUtil.getAvailableBackend()).thenReturn("127.0.0.1:8040");
+        when(mockBackendUtil.getAvailableBackend(anyInt())).thenReturn("127.0.0.1:8040");
+
         backendUtilMockedStatic = mockStatic(BackendUtil.class);
         backendUtilMockedStatic.when(() -> BackendUtil.tryHttpConnection(any())).thenReturn(true);
+        backendUtilMockedStatic
+                .when(() -> BackendUtil.getInstance(any(), any(), any(), any()))
+                .thenReturn(mockBackendUtil);
     }
 
     @Test