Fix internal return types

lib/command.js

@@ -387,7 +387,9 @@ class Command extends EventEmitter {
 
   _hasImplicitHelpCommand() {
     if (this._addImplicitHelpCommand === undefined) {
-      return this.commands.length && !this._actionHandler && !this._findCommand('help');
+      return Boolean(
+        this.commands.length && !this._actionHandler && !this._findCommand('help')
+      );
     }
     return this._addImplicitHelpCommand;
   }
@@ -1352,7 +1354,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
    * Return an option matching `arg` if any.
    *
    * @param {string} arg
-   * @return {Option}
+   * @return {Option | undefined}
    * @api private
    */
 
@@ -1814,7 +1816,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
    * @param {string} str
    * @param {string} [flags]
    * @param {string} [description]
-   * @return {this | string} `this` command for chaining, or version string if no arguments
+   * @return {this | string | undefined} `this` command for chaining, or version string if no arguments
    */
 
   version(str, flags, description) {