Message about "only one entity allowed" only displayed if the selector has square brackets

The bug:
In some places commands require the selector to be able to target only one entity. When the selector doesn't include the square brackets (i.e @e) the message above the chat bar will not display. As opposed to when the selector has square brackets (i.e @e[]). The user should be notified regardless of there being square brackets or not.

To reproduce:
1. Type in chat: /execute if score @s someScore = @e someScore
2. Notice no message displayed above the chat bar.
3. Type in chat: /execute if score @s someScore = @e[]
(with space after the selector)
4. Notice the message about only one entity allowed appear.