Revert "blua: handle 0 length strings in hash_sparse"

This reverts commit c582767603.

src/blua/lstring.c

@@ -90,9 +90,6 @@ static unsigned hash_sparse(const char *str, size_t len)
 {
   /* Constants taken from lookup3 hash by Bob Jenkins. */
   unsigned int a, b, h = cast(unsigned int, len);
-
-  if (len == 0) return 0;
-
 #define rol(x, n) (((x)<<(n)) | ((x)>>(-cast(int, n)&(8*sizeof(x)-1))))
   if (len >= 4) {  /* Caveat: unaligned access! */
     a  = getu32(str);