COPY TO STDOUT shouldn't put None where a function is expected

using the command was failing like the following
```
cassandra@cqlsh> COPY system_schema.tables TO STDOUT ;
'NoneType' object is not callable
cassandra@cqlsh>
```

the logic was not working as expected, and `self.printmsg` was
assigned with `None`

* introduced a unit test to cover this case

Ref: scylladb/scylla-enterprise#3940

pylib/cqlshlib/copyutil.py

@@ -86,6 +86,10 @@ def printmsg(msg, eol='\n'):
     sys.stdout.flush()
 
 
+def noop(*arg, **kwargs):
+    pass
+
+
 class OneWayPipe(object):
     """
     A one way pipe protected by two process level locks, one for reading and one for writing.
@@ -259,7 +263,7 @@ def __init__(self, shell, ks, table, columns, fname, opts, protocol_version, con
             DEBUG = True
 
         # do not display messages when exporting to STDOUT unless --debug is set
-        self.printmsg = printmsg if self.fname is not None or direction == 'from' or DEBUG else None
+        self.printmsg = printmsg if self.fname is not None or direction == 'from' or DEBUG else noop
         self.options = self.parse_options(opts, direction)
 
         self.num_processes = self.options.copy['numprocesses']

pylib/cqlshlib/test/test_copyutil.py

@@ -28,6 +28,9 @@
 from cqlshlib.copyutil import ExportTask
 
 
+Default = object()
+
+
 class CopyTaskTest(unittest.TestCase):
 
     def setUp(self):
@@ -60,10 +63,11 @@ def mock_shell(self):
 
 class TestExportTask(CopyTaskTest):
 
-    def _test_get_ranges_murmur3_base(self, opts, expected_ranges):
+    def _test_get_ranges_murmur3_base(self, opts, expected_ranges, fname=Default):
         """
         Set up a mock shell with a simple token map to test the ExportTask get_ranges function.
         """
+        fname = self.fname if fname is Default else fname
         shell = self.mock_shell()
         shell.conn.metadata.partitioner = 'Murmur3Partitioner'
         # token range for a cluster of 4 nodes with replication factor 3
@@ -77,7 +81,7 @@ def _test_get_ranges_murmur3_base(self, opts, expected_ranges):
         overridden_opts = dict(self.opts)
         for k, v in opts.items():
             overridden_opts[k] = v
-        export_task = ExportTask(shell, self.ks, self.table, self.columns, self.fname, overridden_opts, self.protocol_version, self.config_file)
+        export_task = ExportTask(shell, self.ks, self.table, self.columns, fname, overridden_opts, self.protocol_version, self.config_file)
         assert export_task.get_ranges() == expected_ranges
         export_task.close()
 
@@ -114,3 +118,6 @@ def test_get_ranges_murmur3(self):
             (None, MIN_LONG + 1): {'hosts': ('10.0.0.2', '10.0.0.3', '10.0.0.4'), 'attempts': 0, 'rows': 0, 'workerno': -1}
         }
         self._test_get_ranges_murmur3_base({'endtoken': MIN_LONG + 1}, expected_ranges)
+
+    def test_exporting_to_std(self):
+        self._test_get_ranges_murmur3_base({'begintoken': MIN_LONG - 1}, {}, fname=None)