Address PR comments and add unittest

Author: Theofilos Manitaras

reframe/frontend/cli.py

@@ -771,9 +771,9 @@ def restrict_logging():
         else:
             # Create a unique value to differentiate between configuration
             # parameters with value `None` and invalid ones
-            default = time.time()
+            default = {'token'}
             value = rt.get_option(config_param, default)
-            if value == default:
+            if value is default:
                 printer.error(
                     f'no such configuration parameter found: {config_param}'
                 )

unittests/test_cli.py

@@ -620,6 +620,17 @@ def test_show_config_unknown_param(run_reframe):
     assert returncode == 0
 
 
+def test_show_config_null_param(run_reframe):
+    returncode, stdout, stderr = run_reframe(
+        more_options=['--show-config=general/report_junit'],
+        system='testsys'
+    )
+    assert 'null' in stdout
+    assert 'Traceback' not in stdout
+    assert 'Traceback' not in stderr
+    assert returncode == 0
+
+
 def test_verbosity(run_reframe):
     returncode, stdout, stderr = run_reframe(
         more_options=['-vvvvv'],