prevent crash when clicking unknown item in the recipe

[yl-whosit]

Got this crash when someone clicked an unknown item in the broken recipe:

ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod '' in callback on_playerReceiveFields(): ...mods/unified_inventory/register.lua:332: attempt to index local 'item_def' (a nil value)
 ERROR[Main]: stack traceback:
 ERROR[Main]: .../mods/unified_inventory/register.lua:332: in function 'get_formspec'
 ERROR[Main]: .../mods/unified_inventory/internal.lua:309: in function 'get_formspec'
 ERROR[Main]: .../mods/unified_inventory/internal.lua:334: in function 'set_inventory_formspec'
 ERROR[Main]: .../mods/unified_inventory/callbacks.lua:209: in function 'func'
 ERROR[Main]: .../builtin/profiler/instrumentation.lua:124: in function <.../builtin/profiler/instrumentation.lua:117>
 ERROR[Main]: .../builtin/common/register.lua:26: in function <.../builtin/common/register.lua:12>

unknown items should not exist normally, but it's also better to not crash at least :)

[SmallJoker]

Regression from e071a01 @Emojigit