fix docs and null handling

Signed-off-by: Ritvi Bhatt <ribhatt@amazon.com>

Author: ritvibhatt

core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java

@@ -2483,6 +2483,11 @@ public RelNode visitCluster(
       org.opensearch.sql.ast.tree.Cluster node, CalcitePlanContext context) {
     visitChildren(node, context);
 
+    // Filter out rows where the source field is null before clustering.
+    RexNode sourceFieldRex = rexVisitor.analyze(node.getSourceField(), context);
+    context.relBuilder.filter(
+        context.rexBuilder.makeCall(SqlStdOperatorTable.IS_NOT_NULL, sourceFieldRex));
+
     // Resolve clustering as a window function over all rows (unbounded frame).
     // The window function buffers all rows, runs the greedy clustering algorithm,
     // and returns an array of cluster labels (one per input row, in order).

core/src/main/java/org/opensearch/sql/calcite/udf/udaf/ClusterLabelAggFunction.java

@@ -7,7 +7,6 @@
 
 import java.util.ArrayList;
 import java.util.List;
-import java.util.Objects;
 import org.opensearch.sql.calcite.udf.UserDefinedAggFunction;
 import org.opensearch.sql.common.antlr.SyntaxCheckException;
 import org.opensearch.sql.common.cluster.TextSimilarityClustering;
@@ -41,6 +40,12 @@ public Object result(Acc acc) {
 
   @Override
   public Acc add(Acc acc, Object... values) {
+    // Handle case where Calcite calls generic method with null field value
+    if (values.length == 1) {
+      String field = (values[0] != null) ? values[0].toString() : null;
+      return add(acc, field);
+    }
+
     throw new SyntaxCheckException(
         "Unsupported function signature for cluster aggregate. Valid parameters include (field:"
             + " required string), (t: optional double threshold 0.0-1.0, default 0.8), (match:"
@@ -58,17 +63,17 @@ public Acc add(
       String delims,
       int bufferLimit,
       int maxClusters) {
-    if (Objects.isNull(field)) {
-      return acc;
-    }
+    // Process all rows, even when field is null - convert null to empty string
+    // This ensures the result array matches input row count
+    String processedField = (field != null) ? field : "";
 
     this.threshold = threshold;
     this.matchMode = matchMode;
     this.delims = delims;
     this.bufferLimit = bufferLimit;
     this.maxClusters = maxClusters;
 
-    acc.evaluate(field);
+    acc.evaluate(processedField);
 
     if (bufferLimit > 0 && acc.bufferSize() == bufferLimit) {
       acc.partialMerge(threshold, matchMode, delims, maxClusters);