List table: hidden columns return unnecessary empty items

[afercia]

While investigating on List Tables, noticed the hidden columns (see get_hidden_columns()) returned, for example, this:

where the serialized array stored in the DB has 4 empty strings (going to break it in multiple lines for readability):

`a:8:{
  i:0;s:6:"author";
  i:1;s:10:"categories";
  i:2;s:4:"tags";
  i:3;s:8:"comments";
  i:4;s:0:"";
  i:5;s:0:"";
  i:6;s:0:"";
  i:7;s:0:"";
}`

Turns out the current jQuery selector targets also the table headers in the footer and they don't have an ID. Since we need just the table headers with an ID, and we can't predict if they will be the ones in the thead or the ones in the tfoot, the proposed patch changes the jQuery selector to target just the ones with an ID.
Stored value with the patch applied:

`a:4:{i:0;s:6:"author";i:1;s:10:"categories";i:2;s:4:"tags";i:3;s:8:"comments";}`

[afercia]

Updated patch with coding standards.

[wonderboymusic]

There's another piece to this, wp_ajax_hidden_columns() has to be rebooted to not explode on empty string, which will produce an array with one empty item.

[wonderboymusic]

In 32751:

In wp_ajax_hidden_columns(), don't explode() on an empty string.
In columns.hidden(), which lives in common.js, don't return items with no id.

This was resulting in options like manageedit-postcolumnshidden containing a serialized array with random empty items.

Props afercia, wonderboymusic.
Fixes #32466.

[wonderboymusic]

In 33342:

Revert [32466], I will pick this up in 4.4.

See #32466.

[wonderboymusic]

oops, it's #31846