Check for non-empty array key 1 before using it with current_user_can()

[remyperona]

Check for non-empty array key 1 before using it with current_user_can() to avoid PHP warning

Trac ticket: https://core.trac.wordpress.org/ticket/56254

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[jrfnl]

Hiya, thanks for this PR!

Couple of questions:

1. Are there any tests covering this functionality ? Could a test be added to verify the pre-PHP 8.0 behaviour of the function and to safeguard that the current fix doesn't constitute a BC-break ?
2. The currently proposed fix will only run the current_user_can() check when $sub_item[1] is non-empty and if the answer is false, it will ignore the submenu item.
   This will protect against the "undefined array index" notice, but I'm not sure it constitutes the correct behaviour.
   As current_user_can() is a security function, I wonder if the condition should be changed to:

   if ( empty( $sub_item[1] ) || ! current_user_can( $sub_item[1] ) )

   That would ignore a $sub_item if no capabilities have been set, as well as when the capabilities don't match those of the current user.

I'm honestly not sure what the correct behaviour should be, but adding a test which would codify the behaviour on PHP < 8.0, would confirm the issue on PHP 8.0+ and would safeguard the fix, should be able to clarify this.