Vulcan TKL: fix `table index is nil` exception

[shamil]

Fixes the folowing exception

Lua error in file /usr/share/eruption/scripts/macros.lua: runtime error: ...n/scripts//lib/hwdevices/keyboards/roccat_vulcan_tkl.lua:91: table index is nil
                                                stack traceback:
                                                        [C]: in metamethod '__newindex'
                                                        ...n/scripts//lib/hwdevices/keyboards/roccat_vulcan_tkl.lua:91: in function 'device_specific_key_highlights'
                                                        /usr/share/eruption/scripts//lib/macros/user-macros.lua:293: in function 'update_color_state'
                                                        [string "eruption/src/scripting/script.rs:249:40"]:716: in function 'on_tick'
                                                        UnknownError

[shamil]

I see no interest, closing...