If Java and JNI are found, following code is executed in connect/CMakeLists.txt

   add_jar(JdbcInterface ${JAVA_SOURCES})

    INSTALL(FILES ${CMAKE_CURRENT_SOURCE_DIR}/JavaWrappers.jar

      ${CMAKE_CURRENT_BINARY_DIR}/JdbcInterface.jar

DESTINATION ${INSTALL_PLUGINDIR} COMPONENT connect-engine)

This happens unconditionally also if the connect engine is NOT build.
A good style that helps to avoid these kinds of problems is to do MYSQL_ADD_PLUGIN early and do RETURN() if plugin is not built