Fix ChargeRate.valid_at for rates no duration_from.
Fix ChargeRate.valid_at for rates no duration_from.

hauledev/bookyt,gaapt/bookyt,huerlisi/bookyt,silvermind/bookyt,hauledev/bookyt,huerlisi/bookyt,silvermind/bookyt,huerlisi/bookyt,gaapt/bookyt,xuewenfei/bookyt,xuewenfei/bookyt,wtag/bookyt,hauledev/bookyt,wtag/bookyt,gaapt/bookyt,silvermind/bookyt,hauledev/bookyt,wtag/bookyt,xuewenfei/bookyt,silvermind/bookyt,gaapt/bookyt