Don't call Manager::addProvider(null) upon optional NoSuchFileException

Change-Id: I62e953c886886158f227362fef7048459192217b
(cherry picked from commit 49fa300703643a2e49df01944f1b1299138010c7)
Reviewed-on: https://gerrit.libreoffice.org/6637
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>