PHP 7.4 compatibility / defensive coding

[jrfnl]

WPCS is running into two PHP 7.4 issues where we use array access on a non-array value.
This PR fixes both.

Refs:

• https://wiki.php.net/rfc/deprecate_curly_braces_array_access
• https://wiki.php.net/rfc/notice-for-non-valid-array-container

Build showing the issues:

• https://travis-ci.com/WordPress/WordPress-Coding-Standards/jobs/218025587

[jrfnl]

FYI: just in case anyone is now wondering "but hang on, the PHP 7.4 builds still aren't passing". That's correct and caused by PHP deprecation notices related to PHPCS code upstream.

There is already a PR open upstream to fix these: squizlabs/PHP_CodeSniffer#2560

And for the stable PHPCS versions, I have a PR ready & waiting which will clean up the Travis output and make it more usable for these situations.
Travis test output for that PR combined with this one: https://travis-ci.com/WordPress/WordPress-Coding-Standards/builds/119936162
As you can see, with that combination, the PHP 7.4 build against stable PHPCS is passing.

[jrfnl]

The upstream fixes have been merged, so I restarted the build and the PHP 7.4 / PHPCS master build is now passing: https://travis-ci.com/WordPress/WordPress-Coding-Standards/jobs/218028556