fixes issue when calling save_config() and transport is nxapi (#17835)

An unhandled exeception is raised with using nxapi transport and setting
the save argument to true.  This fix will allow the configuration to be
saved regardless of the transport.

fixes ansible/ansible-modules-core#5094

lib/ansible/module_utils/nxos.py

@@ -60,7 +60,10 @@ class NxapiConfigMixin(object):
             self.execute(['no checkpoint %s' % checkpoint])
 
     def save_config(self, **kwargs):
-        self.execute(['copy running-config startup-config'])
+        try:
+            self.execute(['copy running-config startup-config'], output='text')
+        except TypeError:
+            self.execute(['copy running-config startup-config'])
 
     def load_checkpoint(self, checkpoint):
         try: