Fix PyTraceBackPrintNode to not pass null to nodes

* Notably null is forbidden for Truffle libraries.

Author: eregon

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java

@@ -1572,7 +1572,7 @@ protected void printException(MaterializedFrame frame, PythonModule sys, Object
                 // only print colon if the str() of the object is not the empty string
                 v = objectStr(frame, value);
                 TruffleString s = tryCastToString(v);
-                if (v == null) {
+                if (v == PNone.NONE) {
                     fileWriteString(frame, out, ": <exception str() failed>");
                 } else if (!PGuards.isString(v) || (s != null && !s.isEmpty())) {
                     fileWriteString(frame, out, ": ");

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/lib/PyTraceBackPrintNode.java

@@ -109,7 +109,7 @@ public static boolean fileWriteObject(VirtualFrame frame, Object file, Object da
         } else {
             value = objectRepr(frame, data);
         }
-        if (value == null) {
+        if (value == PNone.NONE) {
             return false;
         }
         fileWriteString(frame, file, castToString(value));
@@ -142,7 +142,7 @@ public static Object objectStr(VirtualFrame frame, Node inliningTarget, Object v
         try {
             return strAsObjectNode.execute(frame, inliningTarget, value);
         } catch (PException pe) {
-            return null;
+            return PNone.NONE;
         }
     }