Changes

osmo-release: debian_patch_apply: handle empty dir

Don't fail with the following bogus error message if the debian/patches
directory exists but is empty:

error: can't open patch '…/git/libosmocore/debian/patches/*.patch': No such file or directory

Fixes: OS#6873
Change-Id: I6656602c58fd221c8bb933890712d1392a1dfdab