fix warning condition on notebook startup

should have only warned if file_to_run is defined *and* it's outside the notebook_dir,
but warning was shown even if it was not specified.

IPython/html/notebookapp.py

@@ -715,7 +715,8 b' class NotebookApp(BaseIPythonApplication):'
             
             nbdir = os.path.abspath(self.notebook_manager.notebook_dir)
             f = self.file_to_run
-            if f and f.startswith(nbdir):
+            if f:
+                if f.startswith(nbdir):
                 f = f[len(nbdir):]
             else:
                 self.log.warn(