core/gatt-client: Don't invalidate Value if length is 0

Reading 0 bytes shall be valid so it shouldn't invalidate the property.
diff --git a/src/gatt-client.c b/src/gatt-client.c
index 57a1f66..114981c 100644
--- a/src/gatt-client.c
+++ b/src/gatt-client.c
@@ -184,7 +184,7 @@
 {
 	gboolean *ret = user_data;
 
-	if (err || length == 0) {
+	if (err) {
 		*ret = FALSE;
 		return;
 	}