Add `match()` helper method on abstract scraper

This is a convenient wrapper around the native preg_match() function that can be used in an OOP way.
Make the scrapers use the `match()` helper method
[fix] existing keys with null values were accepted

If the array of placeholder values had existing keys with null values associated with them, these keys were wrongly considered as valid.

With this change, the method now checks both that the key exists and that the value is not null.