Fix error with toolgun when first opening the spawnmenu

[Guthen]

Error log

Clientside:

[guthscpbase-master] addons/guthscpbase-master/lua/weapons/gmod_tool/stools/guthscp_zone_configurator.lua:125: attempt to index local 'tool' (a nil value)
1. ControlPanelBuildFunction - addons/guthscpbase-master/lua/weapons/gmod_tool/stools/guthscp_zone_configurator.lua:125
2. FillViaTable - gamemodes/sandbox/gamemode/spawnmenu/controlpanel.lua:126
3. ActivateTool - lua/includes/modules/spawnmenu.lua:116
4. Open - gamemodes/sandbox/gamemode/spawnmenu/spawnmenu.lua:130
5. Run - gamemodes/sandbox/gamemode/spawnmenu/spawnmenu.lua:265
6. unknown - gamemodes/base/gamemode/cl_spawnmenu.lua:11
7. unknown - lua/includes/modules/concommand.lua:54

Developper Hints

I believe this happens when first opening the spawnmenu with the tool selected. For some reason (either the tool weapon isn't stored in the player's inventory or the tool mode is wrong), Player:GetTool returns a nil value. Just adding a default value if tool is invalid should fix the error. If it's inconvenient, we can replace the code by directly retrieving the value of the zone_id convar without passing through the tool class.