Fix NullPointerException in PartialDimExtractionFn by explicity checking for dimValue == null

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

@@ -58,6 +58,7 @@ public class PartialDimExtractionFn implements DimExtractionFn
   @Override
   public String apply(String dimValue)
   {
+    if (dimValue == null) return null;
     Matcher matcher = pattern.matcher(dimValue);
     return matcher.find() ? dimValue : null;
   }

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

@@ -41,6 +41,7 @@ public class PartialDimExtractionFnTest
       "Vancouver",
       "Pretoria",
       "Wellington",
+      null,
       "Ontario"
   };