Null partition keys break Hive partition pruning
If the table has a partition key like `key=__HIVE_DEFAULT_PARTITION__`, the following query:

```
SELECT 
  key
, count(*)
from TABLE
GROUP BY 1
```

returns results like:

```
   key   | _col1  
---------+--------
 windows | 1 
 NULL    | 2 
 ios     | 3 
 android | 4 
(4 rows)
```

But the following query return no rows:

```
SELECT 
  key
, count(*)
from TABLE
WHERE key is null 
GROUP BY 1
```

diff --git a/presto-hive/src/main/java/com/facebook/presto/hive/HiveUtil.java b/presto-hive/src/main/java/com/facebook/presto/hive/HiveUtil.java
index 5291b65c4c..a53f35414e 100644
--- a/presto-hive/src/main/java/com/facebook/presto/hive/HiveUtil.java
+++ b/presto-hive/src/main/java/com/facebook/presto/hive/HiveUtil.java
@@ -62,6 +62,7 @@ import java.util.concurrent.Callable;
 import static com.facebook.presto.hive.HiveColumnHandle.hiveColumnIndexGetter;
 import static com.facebook.presto.hive.HiveColumnHandle.isPartitionKeyPredicate;
 import static com.facebook.presto.hive.HiveErrorCode.HIVE_CANNOT_OPEN_SPLIT;
+import static com.facebook.presto.hive.HivePartitionKey.HIVE_DEFAULT_DYNAMIC_PARTITION;
 import static com.facebook.presto.hive.HiveType.HIVE_BOOLEAN;
 import static com.facebook.presto.hive.HiveType.HIVE_BYTE;
 import static com.facebook.presto.hive.HiveType.HIVE_DATE;
@@ -321,7 +322,7 @@ public final class HiveUtil
 
     public static SerializableNativeValue parsePartitionValue(String partitionName, String value, HiveType hiveType, DateTimeZone timeZone)
     {
-        boolean isNull = isHiveNull(value.getBytes(UTF_8));
+        boolean isNull = HIVE_DEFAULT_DYNAMIC_PARTITION.equals(value);
 
         if (HIVE_BOOLEAN.equals(hiveType)) {
             if (isNull) {
['presto-hive/src/main/java/com/facebook/presto/hive/HiveUtil.java']