BUGFIX: check for None moved earlier.

If records is None, then format_records still tries to iterate over
it, causing an exception.  This PR moves the check for None earlier.

IPython/core/ultratb.py

@@ -1026,10 +1026,11 b' class VerboseTB(TBTools):'
         head = self.prepare_header(etype, self.long_header)
         records = self.get_records(etb, number_of_lines_of_context, tb_offset)
 
-        frames = self.format_records(records)
         if records is None:
             return ""
 
+        frames = self.format_records(records)
+
         formatted_exception = self.format_exception(etype, evalue)
         if records:
             filepath, lnum = records[-1][1:3]