libnm: normalize empty strings in 802-1x setting

Supplicant does not allow setting certain properties to empty values.
It also does not make sense.

Also, ifcfg-rh writer uses svSetValueStr() for these properties, so
the ifcfg plugin would always loose having hte values set to "".

Also, you couldn't enter these strings in nmcli.

It's fair to assume that it makes no sense to have these values set to
an empty value. Since we cannot just tighten up verification to reject
them, normalize them.

It also seems that some GUI now starts setting domain_suffix_match to an
empty string. Or maybe it was always doing it, and ifcfg plugin just hid
the problem? Anyway, we have users out there who set these properties to
"". ## 973
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