diff-delta.c: allow delta with empty blob.

Delta computation with an empty blob used to punt and returned NULL.
This commit allows creation with empty blob; all combination of
empty->empty, empty->something, and something->empty are allowed.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff-delta.c

@@ -213,7 +213,7 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 	bdrecord_t *brec;
 	bdfile_t bdf;
 
-	if (!from_size || !to_size || delta_prepare(from_buf, from_size, &bdf))
+	if (delta_prepare(from_buf, from_size, &bdf))
 		return NULL;
 	
 	outpos = 0;