PY-26298 EA-107209 Return empty list in case of exception instead of null

It's not the best idea to suppress exceptions (logged) like that, but
PyPackageManager itself works this way at the moment by caching an empty
list of packages after the first error. It also releases the clients of
refreshAndGetPackages() of burden of handling these (presumably rare)
exceptions on their own.

python/src/com/jetbrains/python/packaging/PyPackageUtil.java

@@ -308,6 +308,7 @@ public class PyPackageUtil {
         packagesRef.set(manager.refreshAndGetPackages(false));
       }
       catch (ExecutionException e) {
+        packagesRef.set(Collections.emptyList());
         if (LOG.isDebugEnabled()) {
           e.initCause(callStacktrace);
           LOG.debug(e);