fdo#79384 RTF import: allow { without } in style names

This is not valid, but LO 3.4 supported it, so add a workaround...

(cherry picked from commit 6092e2eba3f74c9632f7862b2368b0fcf7732f85)

Change-Id: Ib8eefdf41fe9801db6f7e5873eaa1436518912fb