Fix ConcatDiskArray size with zero-length parts

[Beforerr]

Problem

ConcatDiskArray counts each zero-length part as length 1:

using DiskArrays, DiskArrays.TestTypes
a = AccessCountDiskArray(collect(1:2); chunksize=(2,))
e = AccessCountDiskArray(Int[]; chunksize=(2,))
size(cat(e, a, e; dims=1))  # (4,), expected (2,)

Reads are then shifted (d[2:3] returns the wrong elements) and writes fail.

Root cause

arraysize_and_startinds uses 0 to mean "no size seen yet for this grid slot" and then runs replace!(sizeall, 0 => 1) to fill slots that only hold unsized MissingTiles. A real zero length can't be told apart from "unset", so it becomes 1. For the same reason, a zero-size part never conflicts with a differently sized part in the same row or column, so the "Array sizes don't form a grid" check doesn't fire.

Fix

Use -1 to mean "unset". Unsized MissingTile slots still default to length 1.