NPE in UnusedImportsRemover when processing files without definitions.

NPE when a source files contains a package line and at least one
import line but not class nor interface definitions (i.e. empty).

Bug: issue 8566.
(cherry picked from commit 04bc38b5070accc4c86504fc390484c21ded2c33)

Change-Id: Id917eb98e44c28a93fd2da69e3a1d9a23993975b
2 files changed