filter-injector.lua:555: attempt to get length of local 'list' (a nil value)

[cheapie]

The filter conversion LBM crashes if run on a filter missing its inventory metadata.

Example output:

ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod 'pipeworks' in callback LuaLBM::trigger(): .../worlds/survival/worldmods/pipeworks/filter-injector.lua:555: attempt to get length of local 'list' (a nil value)
ERROR[Main]: stack traceback:
ERROR[Main]: 	.../worlds/survival/worldmods/pipeworks/filter-injector.lua:555: in function 'put_to_inputinv'
ERROR[Main]: 	.../worlds/survival/worldmods/pipeworks/filter-injector.lua:574: in function 'func'
ERROR[Main]: 	/home/noah/mt/bin/../builtin/profiler/instrumentation.lua:124: in function 'action'
ERROR[Main]: 	/home/noah/mt/bin/../builtin/game/misc.lua:320: in function </home/noah/mt/bin/../builtin/game/misc.lua:304>

This was originally discovered due to a corrupted map (as in it was caused by a corrupted map, it didn't cause the corruption), but an easy way to reproduce it without any corruption is:

1. With pipeworks enabled, use WorldEdit's //s command or similar to place pipeworks:filter or pipeworks:mese_filter.
2. Load the world at least once with pipeworks disabled.
3. Enable pipeworks again and re-load the world. The server will crash when the mapblock containing the filter is loaded.