prevent NPE on classpath problems

I hope this fixes the NPE I'm seeing on a Scala project that has no source folder,
because I always mix up "classpath variables" and the variables for linked resources...

```
java.lang.NullPointerException
at scala.tools.eclipse.ClasspathManagement26357anonfun.apply(ClasspathManagement.scala:405)
at scala.tools.eclipse.ClasspathManagement26357anonfun.apply(ClasspathManagement.scala:404)
at scala.collection.immutable.List.foreach(List.scala:302)
at scala.tools.eclipse.ClasspathManagement.validateBinaryVersionsOnClasspath(ClasspathManagement.scala:404)
at scala.tools.eclipse.ClasspathManagement.checkClasspath(ClasspathManagement.scala:299)
at scala.tools.eclipse.ClasspathManagement.isClasspathValid(ClasspathManagement.scala:199)
at scala.tools.eclipse.ScalaProject.isClasspathValid(ScalaProject.scala:93)
at scala.tools.eclipse.ScalaBuilder.build(ScalaBuilder.scala:59)
at org.eclipse.core.internal.events.BuildManager.run(BuildManager.java:733)
```