Add a hint for empty input and pressing ESC during prompts

Added this hint because it's not obvious for newcomers and because the task requires to check prompt for two different values to give alert('Canceled').

Author: snakecase

1-js/02-first-steps/10-ifelse/4-check-login/task.md

@@ -20,4 +20,6 @@ The schema:
 
 Please use nested `if` blocks. Mind the overall readability of the code.
 
+Hint: passing an empty input to a prompt returns an empty string `''`. Pressing `key:ESC` during a prompt returns `null`.
+
 [demo]