[improve][fn] Set default tenant and namespace for ListFunctions cmd (apache#23881)

(cherry picked from commit 66d1bb0)
(cherry picked from commit 1abbc3c)

Author: jiangpengcheng

pulsar-client-tools-test/src/test/java/org/apache/pulsar/admin/cli/CmdFunctionsTest.java

@@ -632,6 +632,19 @@ public void testListFunctions() throws Exception {
         verify(functions, times(1)).getFunctions(eq(TENANT), eq(NAMESPACE));
     }
 
+    @Test
+    public void testListFunctionsWithDefaultValue() throws Exception {
+        cmd.run(new String[] {
+                "list",
+        });
+
+        ListFunctions lister = cmd.getLister();
+        assertEquals("public", lister.getTenant());
+        assertEquals("default", lister.getNamespace());
+
+        verify(functions, times(1)).getFunctions(eq("public"), eq("default"));
+    }
+
     @Test
     public void testStateGetter() throws Exception {
         String key = TEST_NAME + "-key";