code: public boolean isSymbolNull(String name) {
          CommandSymbol s = findSymbol(name);
          if (s == null) return true;
          if (s instanceof VarCommand) {
              VarCommand var = (VarCommand) s;
              Value v = var.getValue(this);
              if (v == null || v.get() == null || v instanceof NilValue) return true;
          }
          return false;
      }
docs: Returns true if the symbol is null or has not been defined.
  
  @param name The symbol's name
  @return true if null