Remove check for Nulls buffer when null_count = 0

Summary:
In PyArrow 6.0, it's legit for an arrow array to have non-null NullBuffer while null_count is zero (see also pytorch/torcharrow#109) :
```
import pyarrow as pa
>>> pa.__version__
'6.0.0'
>>> a = pa.array([1, 2, None, 3])
>>> a = a.fill_null(12)
>>> a.buffers()
[<pyarrow.lib.Buffer object at 0x7fe688222330>, <pyarrow.lib.Buffer object at 0x7fe6680c2ef0>]
```

User also reports similiar issue when converting arrow array reading from Parquet, see pytorch/torcharrow#146 (comment)

Differential Revision: D33836988

fbshipit-source-id: bdb7277671eaf4fc25dfad46ed5bbe3272569ace

velox/vector/arrow/Bridge.cpp

@@ -676,10 +676,6 @@ VectorPtr importFromArrowImpl(
         "Nulls buffer can't be null unless null_count is zero.");
     nulls = wrapInBufferView(
         arrowArray.buffers[0], bits::nbytes(arrowArray.length));
-  } else {
-    VELOX_USER_CHECK_NULL(
-        arrowArray.buffers[0],
-        "Nulls buffer must be nullptr when null_count is zero.");
   }
 
   // String data types (VARCHAR and VARBINARY).