Kotlin build gives warning of no Java classes

[tobad357]

Description of the issue:
If a gradle project only has Kotlin source files the Jib build will complain that

warning: Could not find build output directory '/build/classes/java/main'

Expected behavior:
If Kotlin classes are found then there shouldn't be a warning of no java classes found

Steps to reproduce:

1. Create a gradle project with kotlin source code
2. Build the project
3. run gradle jib

Environment: Mac OS X, gradle 4.9, java 1.8