handle missing stream name in nbformat conversion

It was sometimes possible for this key to be absent.
Even though that's never been officially valid,
it happened, so might as well handle it.

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):