don't export systemserverclasspath_fragment if contents are empty

If a systemserverclasspath_fragment only contains libraries that have a
higher min_sdk_version than the target build release version, then we
should not export the systemserverclasspath_fragment. Before this
change, the fragment was exported with an empty `contents` property
which caused errors after being dropped as a prebuilt.

Bug: 289183551
Test: go test ./sdk
Change-Id: Ifefc6880228e4dd37f5e42b2bda31a83df785375