Handle empty AutoplaceSpecification.control

[robertcampion]

Version 2.0.21 added the following bugfix:

Fixed that agricultural tower would show all nauvis tiles as unplantable. Nauvis grass, dirt and red desert are now specifically plantable for tree seeds and highlight as green. (https://forums.factorio.com/122065)

The fix added an autoplace specification to the tree-plant prototype without a control field (code here). This is allowed since control is optional according to the API. However this causes nil to be passed as a key to the append function here, resulting in a "table index is nil" error.

This fix ignores the plant entity if its autoplace.control field is empty, which restores the previous behavior where tree-plant was ignored.