vm:get_light_data does not accept a buffer

[Kimapr]

Minetest version

5.6.1

Summary

I've been working on a mod that uses VoxelManips and found that vm:get_light_data didn't use the buffer i passed to it, while vm:get_data and vm:get_param2_data did. While this is correctly described in the documentation, it's not something anyone would expect.

Steps to reproduce

local vm = ... -- acquire a voxelmanip
local data,data1,data2={},{},{}
vm:get_data(data) -- `data` is filled with data
vm:get_param2_data(data2) -- `data2` is also filled with data
vm:get_light_data(data1) -- `data1` is NOT filled with data (!!!)
assert(#data1>0) -- fails

[grorp]

Fixed by #12682?