first step to have an emptytile.json to avoid senseless vector files

bitmaptilerequest.js

@@ -117,6 +117,15 @@ BitmapTilerequest.prototype =
 								self.abortRequest('Vectortile could not be created. Aborting.');
 								return;
 							}
+
+							if (data.features.length === 0)
+							{
+								// the vector tile was successfully created, but contains no data
+								// -hardlink to emptytile.json, this saves space and inodes on disk
+								// -serve emptytile.png, no need to render
+								self.tile.debug('Vector tile was empty');
+							}
+
 							self.tile.debug('Vector tile created successfully, saving vector tile...');
 							self.tile.saveVectorData(function(err)
 							{