Merge pull request #7492 from minrk/missing-stream

handle missing stream name in nbformat conversion

IPython/nbformat/v4/convert.py

@@ -186,7 +186,7 b' def upgrade_output(output):'
     elif output['output_type'] == 'pyerr':
         output['output_type'] = 'error'
     elif output['output_type'] == 'stream':
-        output['name'] = output.pop('stream')
+        output['name'] = output.pop('stream', 'stdout')
     return output
 
 def downgrade_output(output):