fix JS dim extraction null handling + test case

processing/src/main/java/io/druid/query/extraction/JavascriptDimExtractionFn.java

@@ -51,7 +51,8 @@ public class JavascriptDimExtractionFn implements DimExtractionFn
           cx = contextFactory.enterContext();
         }
 
-        return Context.toString(fn.call(cx, scope, scope, new String[]{input}));
+        final Object res = fn.call(cx, scope, scope, new String[]{input});
+        return res != null ? Context.toString(res) : null;
       }
     };
   }

processing/src/test/java/io/druid/query/extraction/extraction/JavascriptDimExtractionFnTest.java

@@ -20,6 +20,7 @@
 package io.druid.query.extraction.extraction;
 
 import com.google.common.collect.Iterators;
+import com.google.common.collect.Lists;
 import io.druid.query.extraction.DimExtractionFn;
 import io.druid.query.extraction.JavascriptDimExtractionFn;
 import org.junit.Assert;
@@ -52,6 +53,21 @@ public class JavascriptDimExtractionFnTest
     }
   }
 
+  @Test
+  public void testCastingAndNull()
+  {
+    String function = "function(x) {\n  x = Number(x);\n  if(isNaN(x)) return null;\n  return Math.floor(x / 5) * 5;\n}";
+    DimExtractionFn dimExtractionFn = new JavascriptDimExtractionFn(function);
+
+    Iterator<String> it = Iterators.forArray("0", "5", "5", "10", null);
+
+    for(String str : Lists.newArrayList("1", "5", "6", "10", "CA")) {
+      String res = dimExtractionFn.apply(str);
+      String expected = it.next();
+      Assert.assertEquals(expected, res);
+    }
+  }
+
   @Test
   public void testJavascriptRegex()
   {