Do not prefix wp_title() with spaces if $sep is empty

[SergeyBiryukov]

1. Add wp_title('') to header.php.
2. Notice two spaces before the title:

   <title>  Hello World</title>
   

This was previously brought up in #7114, but the fix suggested there wasn't backward compatible.

Wondering if one space might still be expected by themes. For example, if bloginfo('name') is used before wp_title();

#22653 was marked as a duplicate.