Handle a null backtrace in ThreadManager#setException

* This hapenned in CI for ruby-test-mri-asserts:
    java.lang.RuntimeException: Ruby Thread id=259 from test/mri/tests/ruby/test_io.rb:2916 terminated with internal error:
        at org.graalvm.ruby/org.truffleruby.core.thread.ThreadManager.printInternalError(ThreadManager.java:361)
        at org.graalvm.ruby/org.truffleruby.core.thread.ThreadManager.lambda$uncaughtExceptionHandler$3(ThreadManager.java:240)
        at java.base/java.lang.Thread.dispatchUncaughtException(Thread.java:2901)
    Caused by: java.lang.NullPointerException: Cannot invoke "org.truffleruby.language.backtrace.Backtrace.getRaiseException()" because "exception.backtrace" is null
        at org.graalvm.ruby/org.truffleruby.core.thread.ThreadManager.setException(ThreadManager.java:389)
        at org.graalvm.ruby/org.truffleruby.core.thread.ThreadManager.threadMain(ThreadManager.java:344)
        at org.graalvm.ruby/org.truffleruby.core.thread.ThreadManager.lambda$initialize$4(ThreadManager.java:321)
        at java.base/java.lang.Thread.run(Thread.java:1583)
        at org.graalvm.truffle/com.oracle.truffle.polyglot.PolyglotThread.access$001(PolyglotThread.java:53)
        at org.graalvm.truffle/com.oracle.truffle.polyglot.PolyglotThread$1.execute(PolyglotThread.java:106)
        at org.graalvm.truffle/com.oracle.truffle.polyglot.PolyglotThread$ThreadSpawnRootNode.executeImpl(PolyglotThread.java:140)
        at org.graalvm.truffle/com.oracle.truffle.polyglot.PolyglotThread$ThreadSpawnRootNode.execute(PolyglotThread.java:131)
        at org.graalvm.truffle/com.oracle.truffle.api.impl.DefaultCallTarget.callDirectOrIndirect(DefaultCallTarget.java:85)
        at org.graalvm.truffle/com.oracle.truffle.api.impl.DefaultCallTarget.call(DefaultCallTarget.java:102)
        at org.graalvm.truffle/com.oracle.truffle.polyglot.PolyglotThread.run(PolyglotThread.java:102)
        Suppressed: Attached Guest Language Frames (1)

Author: eregon

src/main/java/org/truffleruby/core/thread/ThreadManager.java

@@ -386,7 +386,7 @@ private void setThreadValue(RubyThread thread, Object value) {
     private void setException(RubyThread thread, RubyException exception, Node currentNode) {
         // We materialize the backtrace eagerly here, as the exception escapes the thread and needs
         // to capture the backtrace from this thread.
-        final RaiseException truffleException = exception.backtrace.getRaiseException();
+        RaiseException truffleException = exception.backtrace == null ? null : exception.backtrace.getRaiseException();
         if (truffleException != null) {
             TruffleStackTrace.fillIn(truffleException);
         }