Fix node-inject-nmi to pass an empty body

Fix a request of inject_nmi API to pass an empty body because PUT
request must be a JSON dict. A request without a body causes a
validation error in the server.

Change-Id: Iaaa9e88b4a4b1e1086217900480fb528bfae56dc
Partial-Bug: #1526226

ironicclient/tests/unit/v1/test_node.py

@@ -1221,7 +1221,7 @@ class NodeManagerTest(testtools.TestCase):
         self.mgr.inject_nmi(NODE1['uuid'])
         expect = [
             ('PUT', '/v1/nodes/%s/management/inject_nmi' % NODE1['uuid'],
-             {}, None),
+             {}, {}),
         ]
         self.assertEqual(expect, self.api.calls)
 

ironicclient/v1/node.py

@@ -425,7 +425,7 @@ class NodeManager(base.CreateManager):
 
     def inject_nmi(self, node_uuid):
         path = "%s/management/inject_nmi" % node_uuid
-        return self.update(path, None, http_method='PUT')
+        return self.update(path, {}, http_method='PUT')
 
     def get_supported_boot_devices(self, node_uuid):
         path = "%s/management/boot_device/supported" % node_uuid