If a conversation has no interactions (which shouldn't happen normally,
but sometimes occurs in practice), then its LastInteractionTimeStamp
will be zero, which causes the last interaction date in the smartlist to
be wrongly displayed as 31/12/1969 or 1/1/1970. This patch adds a check
to prevent this.

GitLab: #1794
Change-Id: I1384d6675c9fcaa1904bb6e1706589305b7618e9