android: fix bug with empty directories

Empty directories were not "navigatable-to" (given their empty
contents). However, not showing anything made it appear the
UI entry was unresponsive.

This fixes it an allows the UI to show an empty list of files,
as expected.

Change-Id: Ie07ee72a89ad8ddff139c77d258a04159ffae217
diff --git a/app/src/main/java/io/upspin/upspin/ListDir.java b/app/src/main/java/io/upspin/upspin/ListDir.java
index 9aa763a..d377e6c 100644
--- a/app/src/main/java/io/upspin/upspin/ListDir.java
+++ b/app/src/main/java/io/upspin/upspin/ListDir.java
@@ -220,15 +220,13 @@
 
             @Override
             public void onPostExecute(DirEntry entry) {
-                if (entry != null) {
-                    mAdapter.showEntries(dir, entry);
-                    setCurrentDir(dir);
-                } else {
-                    if (opException != null) {
-                        Snackbar.make(getCurrentFocus(), "Error in Glob:" + opException.getMessage(),
-                                Snackbar.LENGTH_LONG).setAction("Action", null).show();
-                    }
+                if (opException != null) {
+                    Snackbar.make(getCurrentFocus(), "Error in Glob:" + opException.getMessage(),
+                            Snackbar.LENGTH_LONG).setAction("Action", null).show();
+                    return;
                 }
+                mAdapter.showEntries(dir, entry);
+                setCurrentDir(dir);
             }
         }.execute();
     }