Ignore missing prebuilt_apis of java_sdk_library

Building java_sdk_library without defining prebuilt_apis has been
failing with weird error messages. So one have to touch empty txt files
and create prebuilt_apis module, even when the java_sdk_library is
brand-new and has no previous versions. This commit fixes it so that API
check against previous version is skipped, in the case of missing
prebuilt_apis.

Note that Current api txt files (placed under api/ directory) are still
needed (make update-api)

Bug: 126259114
Test: tries to build without touching empty api txt files.
Change-Id: I93630f4139cbf502621693ec315dc06c0d07d1c3