Assembler-constructed torpedoes can end up with null warheads

[benbot16]

Round ID:

7393

Testmerges:

Reproduction:

This occurred on the Serendipity with the slow conveyors being replaced with normal conveyors. We were attempting to assemble AP torpedoes. It happened somewhat intermittently.

Relevant runtime:
[2024-04-28 19:35:29.212] runtime error: Cannot read null.build_path

proc name: new torpedo (/obj/item/ship_weapon/ammunition/torpedo/torpedo_casing/proc/new_torpedo)
source file: nsv13/code/modules/munitions/ammunition/torpedos/torpedo_construction.dm,293
usr: null
src: the NTB-M4A1-IB prebuilt torpe... (/obj/item/ship_weapon/ammunition/torpedo/torpedo_casing)
src.loc: the floor (138,128,2) (/turf/open/floor/plasteel/dark)