Fix build error for no-ramdisk targets

In some build targets, e.g., aosp_arm64_ab, $OUT/ramdisk is an empty
dir, and leads to rsync $OUT/ramdisk/* failure. Removing the trailing
asterisk to avoid throwing an error if it's empty. Note that the
trailing slash still needs to be kept to avoid creating an additional
directory level at the destination.

Bug: 126493225
Test: `make ramdisk_debug` on aosp_arm64_ab
Test: flash boot-debug.img on crosshatch still can adb root
Change-Id: I44937324379fa78fc26a4471ba94eb7694911c2d
(cherry picked from commit 85f292939647fb9bd659a6a5deebd3f07dd3a079)