Allow PlaceholderMessage to be text-less

Summary:
Requested by @iasensio for the KWin Rules KCM patch (D29141). Could be useful for other
uses too, e.g. an app loading screen where you would define an icon and a custom progress
bar, but wouldn't need text.

Test Plan:

Reviewers: iasensio, mart, Kirigami

Reviewed By: iasensio

Subscribers: broulik, iasensio, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D29152