DocxAttributeOutput::CharEscapement: avoid writing <w:sz> twice

As comments-nested.odt shows, that would lead to invalid output.

Change-Id: I5c8f7ae0df60f80b0e58f7007c2f7f5e2a1cee87
fdo#39468 Translate German Comments - sc/cource/filter/dif

Change-Id: I1d44c2538baac9a1443118c95a2886db5098dab1
Reviewed-on: https://gerrit.libreoffice.org/11736
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>