code: @Override
      public String marshall(Object value) {
          if( value == null ) {
              return "null";
          }
          return BuiltInFunctions.getFunction( StringFunction.class ).invoke( value ).cata( justNull(), Function.identity());
      }
docs: Marshalls the give FEEL value into a String. The result is similar to
  calling the string() function in a FEEL expression, with the difference
  that a null value is returned as the "null" string instead of the null
  value itself.
  
  @param value the FEEL value to be marshalled
  
  @return the string representation of the value