allow trimming to an empty database (--keep=0)

IPython/core/historyapp.py

@@ -56,6 +56,7 b' class HistoryTrim(BaseIPythonApplication):'
         
         inputs.pop() # Remove the extra element we got to check the length.
         inputs.reverse()
+        if inputs:
         first_session = inputs[0][0]
         outputs = list(con.execute('SELECT session, line, output FROM '
                                    'output_history WHERE session >= ?', (first_session,)))
@@ -83,6 +84,7 b' class HistoryTrim(BaseIPythonApplication):'
         new_db.commit()
 
 
+        if inputs:
         with new_db:
             # Add the recent history into the new database.
             new_db.executemany('insert into sessions values (?,?,?,?,?)', sessions)