Fix NullFilter getDimensionRangeSet. (apache#15500)

It wasn't checking the column name, so it would return a domain regardless
of the input column. This means that null filters on data sources with range
partitioning would lead to excessive pruning of segments, and therefore
missing results.

processing/src/main/java/org/apache/druid/query/filter/NullFilter.java

@@ -114,6 +114,10 @@ public Filter toFilter()
   @Override
   public RangeSet<String> getDimensionRangeSet(String dimension)
   {
+    if (!Objects.equals(getColumn(), dimension)) {
+      return null;
+    }
+
     RangeSet<String> retSet = TreeRangeSet.create();
     // Nulls are less than empty String in segments
     retSet.add(Range.lessThan(""));

processing/src/test/java/org/apache/druid/segment/filter/NullFilterTests.java

@@ -23,6 +23,8 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.google.common.base.Function;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Range;
+import com.google.common.collect.TreeRangeSet;
 import nl.jqno.equalsverifier.EqualsVerifier;
 import org.apache.druid.common.config.NullHandling;
 import org.apache.druid.jackson.DefaultObjectMapper;
@@ -41,6 +43,7 @@
 
 import java.io.Closeable;
 import java.util.Arrays;
+import java.util.Collections;
 
 @RunWith(Enclosed.class)
 public class NullFilterTests
@@ -310,6 +313,19 @@ public void testArrays()
 
   public static class NullFilterNonParameterizedTest
   {
+    @Test
+    public void testGetDimensionRangeSet()
+    {
+      final NullFilter filter = new NullFilter("x", null);
+
+      Assert.assertEquals(
+          TreeRangeSet.create(Collections.singleton(Range.lessThan(""))),
+          filter.getDimensionRangeSet("x")
+      );
+
+      Assert.assertNull(filter.getDimensionRangeSet("y"));
+    }
+
     @Test
     public void testSerde() throws JsonProcessingException
     {