Resolve regression introduced in [0] intended to fix another regression
introduced in [1] to resolve an assert from [2] (a C++ cleanup).
Recent changes made the array modifier return early with the start-cap's
mesh however this meant the required custom-data layers wouldn't
necessarily exist on the returned mesh, causing selection to crash.
Besides this the same logic was missing from the end-cap.
Resolve by restoring the logic from before [1],
adding a check to prevent the assert.
[0]: 9ad6957574
[1]: 8b2556e8d8
[2]: cb62ab5b28
3acbd93779