Fix "GATT Read Multiple Variable Response" builder

0 length value is perfectly fine, and should result in just length
added into the packet.
Currently, for 0 length value we just break out of loop, and don't add
any value.
This means, that if first characetristic in response had 0 length, we
would return empty packet.

Ignore-AOSP-First: security fix
Test: mma -j32;
Bug: 352696105
Bug: 356886209
Flag: exempt, obvious logic fix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:676d4d0b0bc423fae086f8edf6b9e482780d07d1)
Merged-In: Ida4f6b566cf9fa40fc5330d8084c29669ccaa608
Change-Id: Ida4f6b566cf9fa40fc5330d8084c29669ccaa608

system/stack/gatt/gatt_sr.cc

@@ -198,7 +198,7 @@ static void build_read_multi_rsp(tGATT_SR_CMD* p_cmd, uint16_t mtu) {
 
       len = std::min((size_t) p_rsp->attr_value.len, mtu - total_len);
 
-      if (len == 0) {
+      if (total_len == mtu && p_rsp->attr_value.len > 0) {
         VLOG(1) << "Buffer space not enough for this data item, skipping";
         break;
       }