There's a lot about hygiene in this thread, so I won't comment on choice of NULL or spaces or whatever. Every company I've worked for wants to do that differently, so just program like the locals as far as that goes.

If you make a library function that calls malloc() (or realloc or any other resource allocator) you should also provide a routine in the same library that calls free(). This is because a program might have multiple allocators and this will introduce subtle bugs. This is a mistake many programmers make and it is a very frustrating bug to face because it won't obviously be in the code.

Something else that I think is worth talking about: va_arg isn't very efficient on amd64 which is a very dominant platform at the moment. You might want to consider my macros-based trick:

#define join_strings(s, args...) ({                          \
  char *_args[] = {args};                                      \
  join_strings_(s,_args,sizeof(_args)/sizeof(char*)); \
})
char *join_strings_(char *s, char **invec, size_t n) {
  size_t slen=strlen(s),outlen=slen*(n-1),lens[n],i;char *out=0,*p;
  for(i=0;i<n;++i)outlen+=lens[i]=strlen(invec[i]);
  out=p=malloc(outlen+1);if(!out)abort();
  memcpy(p,*invec,*lens),p+=*lens;
  for(i=1;i<n;++i)memcpy(p,s,slen),p+=slen,memcpy(p,invec[i],lens[i]),p+=lens[i];
  *p=0;
  return out;
}

This is a lot faster (100,000,000 iterations on an 1.7Ghz i7 takes 23 seconds with your implementation but 12 seconds with mine, and it has other benefits: You don't need a terminating NULL (or you can detect it cheaply) which means there's less risk of another programmer making a mistake with your library. This is the single biggest reason I avoid using sentinels.