fix dump_bin

[lerit]

dump_fix data that not in calendar_list, throw error:

NaT is not in list

if first data with dump_all like:

date,open
2000-01-04,111
2000-01-05,222
2000-01-06,333

and then second with dump_fix like:

date,open
2000-01-01,111
2000-01-02,222
2000-01-03,333

it will cause _df to be empty,and cause error(because df not in calendar_list range):

_df = self.data_merge_calendar(df, calendar_list)

i know even this pull request ,old data still not be update to bin,it just a way to catch and give message for such data

[you-n-g]

Thanks! It looks great.
Welcome to become one of Qlib's contributors