[Bugfix] Fix #2076 (#2111)

* Fix

* Revert "Fix"

This reverts commit 2627630daee33a709a70ef87e1ad9930772430a8.

* Fix

* upd
Co-authored-by: Ubuntu <ubuntu@ip-172-31-68-185.ec2.internal>

python/dgl/heterograph.py

@@ -5279,6 +5279,9 @@ class DGLHeteroGraph(object):
         >>> g.format()
         {'created': ['coo', 'csr', 'csc'], 'not created': []}
         """
+        if self.num_edges() == 0:
+            return 0
+
         return self._graph.create_format_()
 
     def astype(self, idtype):