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