BUG: Fix groupby bug pandas-dev#21624.

Fixes bug where operations such as transform('sum') raise errors when only a single null group exists.

Author: lopez86

pandas/core/groupby/groupby.py

@@ -2350,6 +2350,10 @@ def size(self):
             out = np.bincount(ids[ids != -1], minlength=ngroup)
         else:
             out = ids
+        # Covers the edge case where only a null group exists
+        if self.result_index.shape[0] == 0:
+            out = []
+
         return Series(out,
                       index=self.result_index,
                       dtype='int64')