Most string properties should not accept empty strings. Add a generic
way to reject them during verify.

Add a new flag NMSettInfoProperty.direct_string_allow_empty.

Note that properties must opt-in to allow empty values. Since all
existing properties didn't have this check (but hopefully re-implemented
it in verify()), all existing properties get this flag set to TRUE.

The main point here it that new properties get the strict check by
default.

We should also review existing uses of direct_string_allow_empty,
whether the flag can be cleared. This can be done if verify() already
enforces a non-empty string, or if we accept to break behavior by
tightening up the check.