Avoid NullPointerException in --dumpGraphs & test & docs

[JaroslavTulach]

Pull Request Description

Additional bugfix for #5709 with an improved documentation and test for @BuiltinMethod and its processor.

Checklist

• All code follows the
  Java,
• All code has been tested:
  • Manually via IGV
  • Unit tests written

[JaroslavTulach]

After integration of #6255 the following command no longer works:

sbt:runtime> withDebug --dumpGraphs benchOnly -- ArrayProxyBenchmarks.sumOverDelegatingProxy

it yields org.graalvm.jniutils.JNIExceptionWrapper: java.lang.NullPointerException and the code doesn't get properly compiled.

 Warmup Iteration   1: java.util.concurrent.ExecutionException: java.lang.NullPointerException
        at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122)
        at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191)
        at truffle.runtime.CompilationTask.awaitCompletion(CompilationTask.java:108)
        at truffle.runtime.GraalTruffleRuntime.uninterruptibleWaitForCompilation(GraalTruffleRuntime.java:863)
        at truffle.runtime.GraalTruffleRuntime.finishCompilation(GraalTruffleRuntime.java:839)
        at truffle.runtime.OptimizedCallTarget.maybeWaitForTask(OptimizedCallTarget.java:841)
        at truffle.runtime.OptimizedCallTarget.compile(OptimizedCallTarget.java:833)
        at truffle.runtime.OptimizedCallTarget.interpreterCall(OptimizedCallTarget.java:583)
        at truffle.runtime.OptimizedCallTarget.callBoundary(OptimizedCallTarget.java:562)
        at truffle.runtime.OptimizedCallTarget.doInvoke(OptimizedCallTarget.java:549)
        at truffle.runtime.OptimizedCallTarget.callDirect(OptimizedCallTarget.java:495)
        at truffle.runtime.OptimizedDirectCallNode.call(OptimizedDirectCallNode.java:68)
        at org.enso.interpreter.node.callable.ExecuteCallNode.callDirect(ExecuteCallNode.java:57)
        at org.enso.interpreter.node.callable.ExecuteCallNodeGen.executeCall(ExecuteCallNodeGen.java:42)
        at org.enso.interpreter.node.callable.dispatch.SimpleCallOptimiserNode.executeDispatch(:49)

This is caused by our adventurous subclassing of DirectCallNode and returning null from its getClonnedTarget() method. This PR fixes the problem by returning non-null call target.