Add extra condition to Dumpling.targets import.

Author: mellinoe

dir.traversal.targets

@@ -78,7 +78,7 @@
   </Target>
 
   <!-- Dumpling.targets needs to install dumpling.py before running tests. -->
-  <Import Project="$(ToolsDir)Dumpling.targets" Condition="'$(EnableDumpling)' == 'true' OR '$(EnableCloudTest)' == 'true'" />
+  <Import Project="$(ToolsDir)Dumpling.targets" Condition="Exists('$(ToolsDir)Dumpling.targets') AND ('$(EnableDumpling)' == 'true' OR '$(EnableCloudTest)' == 'true')" />
 
   <Target Name="GetFilesToPackage"
           DependsOnTargets="FilterProjects"