ENH: Allow LIST(APPEND) command to append nothing.

Source/cmListCommand.cxx

@@ -179,12 +179,18 @@ bool cmListCommand::HandleGetCommand(std::vector<std::string> const& args)
 //----------------------------------------------------------------------------
 bool cmListCommand::HandleAppendCommand(std::vector<std::string> const& args)
 {
-  if(args.size() < 3)
+  if(args.size() < 2)
     {
-    this->SetError("sub-command APPEND requires at least two arguments.");
+    this->SetError("sub-command APPEND requires at least one argument.");
     return false;
     }
 
+  // Skip if nothing to append.
+  if(args.size() < 3)
+    {
+    return true;
+    }
+
   const std::string& listName = args[1];
   // expand the variable
   std::string listString;