Checking whether the m_textTexture is not null before dereferencing it

Summary:
It can be null even after updateTextTexture() if there is not text
to generate the texture from:

        if (m_effectFrame->text().isEmpty())
            return;

Reviewers: graesslin

Reviewed By: graesslin

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1347