fdo#74363: fix auto correct of initial capitals on start of first para

The GetPrevPara() method apparently has to return 0 when there is no
previous paragraph.

(regression from ac85b6cff11d193f5f71d11b1f3cc1c474653f59)

Change-Id: I09a3e1d3a3adb33562e4e03c0755447047cbd433
(cherry picked from commit ebd0d0056d89a6a69d2cb75e28758132cb5c236b)
Reviewed-on: https://gerrit.libreoffice.org/7940
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>