vndk-def: Skip ndk ext check if no generic-refs.

After this commit, vndk_definition_tool.py will skip the NDK extension
check if generic references are not specified by the user.  This fixes
an unexpected AttributeError on NoneType object.

Test: Run ./vndk_definition_tool.py vndk command w/o --generic-refs ...
Change-Id: Ibcd960039c5acc4adabb1a2e2b5965a026e9f23a
diff --git a/vndk/tools/definition-tool/vndk_definition_tool.py b/vndk/tools/definition-tool/vndk_definition_tool.py
index 7c16538..92633b7 100755
--- a/vndk/tools/definition-tool/vndk_definition_tool.py
+++ b/vndk/tools/definition-tool/vndk_definition_tool.py
@@ -1598,7 +1598,8 @@
                                  generic_refs=generic_refs)
 
         # Check the API extensions to NDK libraries.
-        self._check_ndk_extensions(graph, generic_refs)
+        if generic_refs:
+            self._check_ndk_extensions(graph, generic_refs)
 
         # Create banned libraries.
         if not args.ban_vendor_lib_dep: