Fix PHP 7.3 warning when using count() on non-array value

[westonruter]

A PHP warning is being raised in PHP 7.3:

PHP Warning: count(): Parameter must be an array or an object that implements Countable in .../sabberworm/php-css-parser/lib/Sabberworm/CSS/Parsing/ParserState.php on line 30

This was reported a couple times in the context of the AMP plugin for WordPress:

• https://wordpress.org/support/topic/php-7-3-9-warning-causing-timeout-on-amp-urls/
• Turn off display_errors during the post-processing phase (sanitization) ampproject/amp-wp#1915 (comment)

The suggested fix is to ensure that count() is only being done with aText when it has been populated with an array value.