nob_cmd_render and nob__cmd_start_process

[fbucafusco]

Fixes nob for when you have this pattern:

#ifdef OPTIM #define CFLAGS "-O2" #else #define CFLAGS "" #endif .... cmd_append(&cmd, COMPILER_C, "-c" , "file.c" , CFLAGS , INCLUDES , "-o", "myexec");

When OPTIM is not defined, then CFLAGS is "" and execvp fails. also works with #define CFLAGS NULL

Also fixes nob_cmd_render.