make it clear to use why anki's suggesting a download

avoid showing the upload button to prevent accidental data loss; user
can add a card if they want to upload

aqt/sync.py

@@ -229,6 +229,12 @@ enter your details below.""") %
         return (u, p)
 
     def _confirmFullSync(self):
+        if self.thread.localIsEmpty:
+            diag = askUserDialog(
+                _("Local collection has no cards. Download from AnkiWeb?"),
+                [_("Download from AnkiWeb"), _("Cancel")])
+            diag.setDefault(1)
+        else:
             diag = askUserDialog(_("""\
 Your decks here and on AnkiWeb differ in such a way that they can't \
 be merged together, so it's necessary to overwrite the decks on one \
@@ -374,13 +380,10 @@ class SyncThread(QThread):
         self._syncMedia()
 
     def _fullSync(self):
-        # if the local deck is empty, assume user is trying to download
-        if self.col.isEmpty():
-            f = "download"
-        else:
         # tell the calling thread we need a decision on sync direction, and
         # wait for a reply
         self.fullSyncChoice = False
+        self.localIsEmpty = self.col.isEmpty()
         self.fireEvent("fullSync")
         while not self.fullSyncChoice:
             time.sleep(0.1)