[string] Check if a string isn't nil or empty in Lua

Can this code be simplified in one if test instead two?

nil and '' are different values. If you need to test that s is neither, IMO you should just compare against both, because it makes your intent the most clear.

That and a few alternatives, with their generated bytecode:

if not foo or foo == '' then end
     GETGLOBAL       0 -1    ; foo
     TEST            0 0 0
     JMP             3       ; to 7
     GETGLOBAL       0 -1    ; foo
     EQ              0 0 -2  ; - ""
     JMP             0       ; to 7

if foo == nil or foo == '' then end
    GETGLOBAL       0 -1    ; foo
    EQ              1 0 -2  ; - nil
    JMP             3       ; to 7
    GETGLOBAL       0 -1    ; foo
    EQ              0 0 -3  ; - ""
    JMP             0       ; to 7

if (foo or '') == '' then end
   GETGLOBAL       0 -1    ; foo
   TEST            0 0 1
   JMP             1       ; to 5
   LOADK           0 -2    ; ""
   EQ              0 0 -2  ; - ""
   JMP             0       ; to 7

The second is fastest in Lua 5.1 and 5.2 (on my machine anyway), but difference is tiny. I'd go with the first for clarity's sake.