Fix stringer rule logic for null superclasses

Author: fee1-dead

src/main/java/com/javadeobfuscator/deobfuscator/rules/stringer/RuleStringDecryptorWithThread.java

@@ -34,6 +34,9 @@ public String getDescription() {
     @Override
     public String test(Deobfuscator deobfuscator) {
         for (ClassNode classNode : deobfuscator.getClasses().values()) {
+            if (classNode.superName == null) {
+                continue;
+            }
             if (!classNode.superName.equals("java/lang/Thread")) {
                 continue;
             }