Improve get_shortcode_regex performance

[Antek88]

Hi,

It's my first PR to WordPress, I hope it goes well :).

I ran Blackfire on my homepage (eCommerce shop with Woocommerce plugin). After analyzing it, I see that many shortcodes are parsing, but I don't use it directly. After some investigation, I see that in
get_shortcode_regex is many unnecessary call implode/array_map function for empty $tagsname argument.
After this patch, page response time is reduced by almost ~200ms.
Blackfire profile screen:

Trac ticket: https://core.trac.wordpress.org/ticket/54289#ticket

[flixos90]

Thank you for the report @Antek88, and welcome to Trac.

While this may be a valid ticket, I think at this point short codes aren't recommended anymore, so I'd consider this low severity. That said, since you have already opened a pull request, it might be straightforward enough to address. Let me take a look.