Prevent access through 'NoneType' when closing activity_stream

Although I'm unable to reproduce the issue, its a safe change to
prevent an AttributeError from occuring ('NoneType' object has no
attribute 'close').  The user that reported this is attempting to
launch a kernel and I believe the launch only partially completed
such that `kernel._activity_stream` did not get established.
(This occurred from Jupyter Enterprise Gateway where we deal with remote
kernel launches across resource-managed clusters, so things are a bit
more involved relative to kernel establishment.)

notebook/services/kernels/kernelmanager.py

@@ -272,8 +272,9 @@ class MappingKernelManager(MultiKernelManager):
         """Shutdown a kernel by kernel_id"""
         self._check_kernel_id(kernel_id)
         kernel = self._kernels[kernel_id]
-        kernel._activity_stream.close()
-        kernel._activity_stream = None
+        if kernel._activity_stream:
+            kernel._activity_stream.close()
+            kernel._activity_stream = None
         self.stop_buffering(kernel_id)
         self._kernel_connections.pop(kernel_id, None)
         self.last_kernel_activity = utcnow()