bugfix: Don't allow empty JAVA_HOME

Author: tgodzik

modules/options/src/main/scala/scala/build/options/BuildOptions.scala

@@ -557,7 +557,7 @@ object BuildOptions {
     def envUpdates(currentEnv: Map[String, String]): Map[String, String] = {
       // On Windows, AFAIK, env vars are "case-insensitive but case-preserving".
       // If PATH was defined as "Path", we need to update "Path", not "PATH".
-      // Same for JAVA_HOME.
+      // Same for JAVA_HOME
       def keyFor(name: String) =
         if (Properties.isWin)
           currentEnv.keys.find(_.equalsIgnoreCase(name)).getOrElse(name)

modules/options/src/main/scala/scala/build/options/JavaOptions.scala

@@ -149,7 +149,7 @@ final case class JavaOptions(
     }
 
   private def findLocalDefaultJava(): Option[Positioned[os.Path]] =
-    Option(System.getenv("JAVA_HOME")).map(p =>
+    Option(System.getenv("JAVA_HOME")).filter(_.nonEmpty).map(p =>
       Positioned(Position.Custom("JAVA_HOME env"), os.Path(p, os.pwd))
     ).orElse(
       sys.props.get("java.home").map(p =>