Handle falsy values in addHelpCommand() properly

Author: aweebit

lib/command.js

@@ -365,7 +365,7 @@ class Command extends EventEmitter {
    */
 
   addHelpCommand(enableOrNameAndArgs, description) {
-    if (enableOrNameAndArgs === false) {
+    if (!enableOrNameAndArgs && enableOrNameAndArgs !== undefined) {
       this._addImplicitHelpCommand = false;
     } else {
       this._addImplicitHelpCommand = true;