apiMappings are incorrect

[mkurz]

In Java 11 the property "sun.boot.class.path" is null, so this can't work anymore:

playframework/project/BuildSettings.scala

Lines 149 to 162 in 58ee880

	apiMappings ++= {
	// Maps JDK 1.8 jar into apidoc.
	val rtJar = sys.props
	.get("sun.boot.class.path")
	.flatMap(cp =>
	cp.split(java.io.File.pathSeparator).collectFirst {
	case str if str.endsWith(java.io.File.separator + "rt.jar") => str
	}
	)
	rtJar match {
	case None => Map.empty
	case Some(rtJar) => Map(file(rtJar) -> url(Docs.javaApiUrl))
	}
	},