Modify error emit condition for checking api files in java_api_library

java_api_library currently emits error based when the
java_api_contribution module has an empty api file or when the module
does not have any api files. This may be problematic in partial
manifests where allow missing dependency is set, thus add this condition
for error emission.

Test: go test ./java
Bug: 284212875
Change-Id: I10b4baa5723e7a53d931823b3029ea20ac731911