basic/strbuf: include empty strings in count

Not that it matters much, but it seems cleaner to also count those
inputs, even if they do not consume extra storage space.

The test is extended to include an empty input and counts in the test are
adjusted to include it.

src/basic/strbuf.c

@@ -133,9 +133,12 @@ ssize_t strbuf_add_string(struct strbuf *str, const char *s, size_t len) {
                 return -EINVAL;
 
         /* search string; start from last character to find possibly matching tails */
-        if (len == 0)
-                return 0;
+
         str->in_count++;
+        if (len == 0) {
+                str->dedup_count++;
+                return 0;
+        }
         str->in_len += len;
 
         node = str->root;