Allow VNDK to be missing from compatibility matrix.

<vndk> is injected at build time.

Test: pass
Change-Id: I91146012cd3e9cbd1e15abb33757490479124efa
diff --git a/parse_xml.cpp b/parse_xml.cpp
index 82e96d3..1502011 100644
--- a/parse_xml.cpp
+++ b/parse_xml.cpp
@@ -690,7 +690,9 @@
                 return false;
             }
         } else if (object->mType == SchemaType::DEVICE) {
-            if (!parseChild(root, vndkConverter, &object->device.mVndk)) {
+            // <vndk> can be missing because it can be determined at build time, not hard-coded
+            // in the XML file.
+            if (!parseOptionalChild(root, vndkConverter, {}, &object->device.mVndk)) {
                 return false;
             }
         }