Do not add empty <vndk> tag to device compatibility matrix

... to make the output clean. This change is backwards compatible
since <vndk> is parsed as an optional tag.

From this change on, <vndk> should no longer be used (both
in device compatibility matrix and in framework manifest).

Test: libvintf_test
Bug: 36400653
Change-Id: I3b5e849dc39350ba9b5782568991544572ed31d5
diff --git a/parse_xml.cpp b/parse_xml.cpp
index 3df7e19..1723bd0 100644
--- a/parse_xml.cpp
+++ b/parse_xml.cpp
@@ -901,7 +901,9 @@
             }
         } else if (m.mType == SchemaType::DEVICE) {
             if (!(flags & SerializeFlag::NO_VNDK)) {
-                appendChild(root, vndkConverter(m.device.mVndk, d));
+                if (!(m.device.mVndk == Vndk{})) {
+                    appendChild(root, vndkConverter(m.device.mVndk, d));
+                }
             }
         }