Provide at least one copy of protos with repeated fields

PR #8 added ExpandRepeatedProto, which introduced support for protos with
repeated fields by providing one copy of a proto for each value in a
repeated field.

Following this logic, if the repeated field had no values, ExpandRepeatedProto
would provide exactly 0 copies. This behavior is undesired, as other
fields might still contain values. This commit therefore ensures that at
least one copy of the proto is returned, even if there aren't any values
in the repeated field.
Merge pull request #14 from nmalkin/repeat-at-least-once

Provide at least one copy of protos with repeated fields