[3.0] Drop the placeholders when flushing the board move stack

[albertlast]

Description

Admin → Boards and Categories → Move draws one link per place the board could go.
While walking the tree, the "after this board" link is held back on a stack until that
board's children have been listed. A board inside the subtree being moved has no links
of its own, so null goes onto the stack in its place, purely to keep the stack as deep
as the tree:

array_push($stack, !empty(…['move_links']) ? array_shift(…['move_links']) : null);

Popping already skips those (if (($temp = array_pop($stack)) != null)). Flushing what is
left at the end of the category does not, so the placeholders go straight into
move_links and reach the template:

<a href="" class="move_links" title=""><span class="main_icons select_" title=""></span></a>

plus two warnings per placeholder, because ManageBoards.template.php reads child_level
off the first entry and href off each one:

2: Trying to access array offset on null  —  ManageBoards.template.php:84
2: Trying to access array offset on null  —  ManageBoards.template.php:88

What it looks like

Moving a top-level board that has children is where it bites. On this tree:

General Discussion
    Child Board
        Grandchild
        Second Top Board
            Placeholder

"Move" on General Discussion offered four destinations, and three of them were the dead
links above — one real choice left on the page.

Checked

array_filter() on the stack before it is flushed. Every move= on that tree, before and
after:

board being moved	links before	of those, dead	links after
General Discussion	4	3	1
Child Board	5	2	3
Grandchild	9	0	9
Second Top Board	8	1	7
Placeholder	9	0	9

Every surviving link is the same one it was before — only the empty ones go. Then actually
moved a board through the UI to confirm the links still work, and the error log stayed
empty across the lot.

Issues References (Fixes|Related|Closes)

n/a