Dear William,
The correct syntax is the following:
[wpv-if testimonial="wpcf-reviews" evaluate="!empty($testimonial)"] [types field="reviews" class="white-text"][/types] <span class="client-name">-[wpv-post-title]</span> [wpv-if]
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Caridad