apacheGH-41590: [Java] Improve BaseRepeatedValueVector function on isEmpty and isNull operations (apache#41601)

### Rationale for this change
Resolves apache#41590 .

### What changes are included in this PR?
Make `abstract` on `isNull` and `isEmpty` of `BaseRepeatedValueVector`.

### Are these changes tested?
Existing tests pass.

### Are there any user-facing changes?
No.

* GitHub Issue: apache#41590

Authored-by: Tai Le Manh <[email protected]>
Signed-off-by: David Li <[email protected]>

Author: tlm365

vector/src/main/java/org/apache/arrow/vector/complex/BaseRepeatedValueVector.java

@@ -355,16 +355,8 @@ public int getInnerValueCountAt(int index) {
             offsetBuffer.getInt(index * OFFSET_WIDTH);
   }
 
-  /** Return if value at index is null (this implementation is always false). */
-  @Override
-  public boolean isNull(int index) {
-    return false;
-  }
-
-  /** Return if value at index is empty (this implementation is always false). */
-  public boolean isEmpty(int index) {
-    return false;
-  }
+  /** Return if value at index is empty. */
+  public abstract boolean isEmpty(int index);
 
   /** Starts a new repeated value. */
   public int startNewValue(int index) {