ucb: fix inverted condition in webdav lcl_sendPartialGETRequest

(regression from a3f32769fc4bb23c64168b412dd10ec769a3854d)

Change-Id: Ic72b0e874ebf19e25e43af1f8750c63c04ed8a8e
(cherry picked from commit 3780738154b8c3b3f9d85c64cccf621d97574886)
Reviewed-on: https://gerrit.libreoffice.org/7581
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>