[minor] following HUDI-4739, fix the extraction for simple record keys (apache#6594)

Author: danny0405

hudi-client/hudi-client-common/src/main/java/org/apache/hudi/keygen/KeyGenUtils.java

@@ -75,7 +75,9 @@ public static String[] extractRecordKeys(String recordKey) {
     String[] fieldKV = recordKey.split(",");
     return Arrays.stream(fieldKV).map(kv -> {
       final String[] kvArray = kv.split(":");
-      if (kvArray[1].equals(NULL_RECORDKEY_PLACEHOLDER)) {
+      if (kvArray.length == 1) {
+        return kvArray[0];
+      } else if (kvArray[1].equals(NULL_RECORDKEY_PLACEHOLDER)) {
         return null;
       } else if (kvArray[1].equals(EMPTY_RECORDKEY_PLACEHOLDER)) {
         return "";

hudi-client/hudi-client-common/src/test/java/org/apache/hudi/keygen/TestKeyGenUtils.java

@@ -25,6 +25,7 @@ public class TestKeyGenUtils {
 
   @Test
   public void testExtractRecordKeys() {
+    // test complex key form: field1:val1,field2:val2,...
     String[] s1 = KeyGenUtils.extractRecordKeys("id:1");
     Assertions.assertArrayEquals(new String[]{"1"}, s1);
 
@@ -33,5 +34,9 @@ public void testExtractRecordKeys() {
 
     String[] s3 = KeyGenUtils.extractRecordKeys("id:1,id2:__null__,id3:__empty__");
     Assertions.assertArrayEquals(new String[]{"1", null, ""}, s3);
+
+    // test simple key form: val1
+    String[] s4 = KeyGenUtils.extractRecordKeys("1");
+    Assertions.assertArrayEquals(new String[]{"1"}, s4);
   }
 }