Allow PHP version check to execute on older versions of PHP

Allow PHP version check to execute on older versions of PHP

A change to update.php used the null coalesce (??) operator.
While this is normally fine, the code in question executes
before the PHP version check, and causes an unfriendly PHP
error rather than the intended helpful error. Use the older-style
isset() call instead, so that the PHP version check will get
a chance to execute.

Bug: T213893
Change-Id: I22e4a24bed9e0b29e08afc7b9468e7bfd81d7d57