From: filters are not correctly represented

[crossan007]

Gmail filters matched only on from: queries are not correctly represented by the program.

Given a filter with only a from criteria I would expect a corresponding entry in the file produced by --export; however, only empty filter sets are produced:

[[Filter]]
Query = ""
Archive = false
Read = false
Delete = false
ToMe = false
ArchiveUnlessToMe = false
Label = ""
ForwardTo = ""

I suspect this is an issue because other criteria do generate values in the export file, such as compound criteria. example:

[[Filter]]
Query = "from:(\"wafflebot[bot]\") OR subject:(Apply fixes from StyleCI)"
Archive = false
Read = false
Delete = false
ToMe = false
ArchiveUnlessToMe = false
Label = "P1/00A2 - Github/Auto"
ForwardTo = ""