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.

Replying to nacin:

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

Themes should not be putting anything other than <?php wp_title(); ?> inside of the HTML <title></title> tags. Adding bloginfo( 'name' ) should be done via wp_title filter, even in Themes. Note that this is enforced in the Theme Review guidelines, so if that use case the only impediment, I would recommend not holding up this patch because of it.