Revert changes to Span will always have a field with the current implementation

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/ast/expression/Span.java

@@ -25,10 +25,7 @@ public class Span extends UnresolvedExpression {
 
   @Override
   public List<UnresolvedExpression> getChild() {
-    if (field == null) {
-      return ImmutableList.of(value);
-    }
-    return List.of(field, value);
+    return ImmutableList.of(field, value);
   }
 
   @Override

ppl/src/main/java/org/opensearch/sql/ppl/utils/PPLQueryDataAnonymizer.java

@@ -729,13 +729,9 @@ public String visitAggregateFunction(AggregateFunction node, String context) {
 
     @Override
     public String visitSpan(Span node, String context) {
-      String field = node.getField() != null ? analyze(node.getField(), context) : null;
+      String field = analyze(node.getField(), context);
       String value = analyze(node.getValue(), context);
-      if (field != null) {
-        return StringUtils.format("span(%s, %s %s)", field, value, node.getUnit().getName());
-      } else {
-        return StringUtils.format("span(%s %s)", value, node.getUnit().getName());
-      }
+      return StringUtils.format("span(%s, %s %s)", field, value, node.getUnit().getName());
     }
 
     @Override