fix crash in some circumstances when debugging using IntelliJ-EmmyLua

[E-Fir]

Problem: MobDebug crashes when debugging using IntelliJ-EmmyLua.
Example:

line:   EXEC return {
line:         get = 'GET_single-senders-senderId';

./apis/nmos.lua:309: /usr/share/lua/5.4/mobdebug.lua:812: bad argument #2 to 'sub' (number expected, got nil) stack traceback:
        [C]: in function 'error'
        /usr/share/lua/5.4/mobdebug.lua:719: in hook '?'
        ...

Solution: wrap string.sub in pcall.