CommandBox: update handleCommandEntered() for empty strings
CommandBox: update handleCommandEntered() for empty strings

CommandBox passes empty string to Logic for parsing and execution.
An empty string is unsuccessfully parsed, resulting in an error message
stating "Invalid command format" being displayed to the user.

This behaviour seems unintuitive, as it is unlikely that users expect
an empty string to be able to execute a correct command.

Let's teach CommandBox to perform nothing when there is no user input
and the enter button is pressed.

Original author: @bchenghi

se-edu/addressbook-level3