User and node "created" dates (from feeds_to_unixtime()) default to the wrong timezone

Issue Summary

If you import a node or user and don't have the "created" date as one of the fields, Feeds will create a default timestamp for this field that is incorrect (basically it seems to assume your local timezone is UTC).

This is closely related to the following issues:
#722740: Feeds Date mapper converts imported dates to GMT unless in UNIX timestamp format
#857216: Behavior on importing empty/NULL/invalid dates

However, it wouldn't be fixed by the patches in either of them. The solution, in any case, should be simple; the code already tries to fall back on REQUEST_TIME in the case where no created date is provided, but feeds_to_unixtime() doesn't let that happen because it attempts to convert a string to a date even when the string is empty.