Crashes when ran on multicraft server, b/c minetest.get_server_status() returns nil.

[dennisjenkins75]

Exact same root cause as minetest-mapserver/mapserver_mod#20

So if minetest.get_server_status() returns nil (which is allowed per the API docs), then the monitoring mod should not crash. Just assume that the lag is 0 or something?

ServerError: AsyncErr: environment_Step: Runtime error from mod 'monitoring' in callback environment_Step(): ...e/1hit/worlds/world/worldmods/monitoring/builtin/lag.lua:12: bad argument #1 to 'gmatch' (string expected, got nil)
stack traceback:
        [C]: in function 'gmatch'
        ...e/1hit/worlds/world/worldmods/monitoring/builtin/lag.lua:12: in function 'explode'
        ...e/1hit/worlds/world/worldmods/monitoring/builtin/lag.lua:20: in function 'get_max_lag'
        ...e/1hit/worlds/world/worldmods/monitoring/builtin/lag.lua:46: in function 'globalstep'
        ...worlds/world/worldmods/monitoring/builtin/globalstep.lua:32: in function <...worlds/world/worldmods/monitoring/builtin/globalstep.lua:24>
        /home/1hit/multicraft/bin/../builtin/game/register.lua:441: in function </home/1hit/multicraft/bin/../builtin/game/register.lua:425>