Better json::null handling for loops

[ludekvodicka]

Currently, when json element is empty and we're trying to iterate it application throws the error.

I believe that in this situations loop should be skipped but not stopped.

Fix is pretty easy, it's necessary to test for null in eval_expression function

	template<typename T = json>
  T eval_expression(const Parsed::ElementExpression& element, const json& data) {
		const json var = eval_function(element, data);
		if (var.is_null())            // << test if element is empty
			return T();           // << return empty

		try {
			return var.get<T>();
		} catch (json::type_error& e) {
			inja_throw("json_error", e.what());
			throw;
		}
	}

The use case is that I'm filling database data to the json context and generating some stats from such data. But when array of data is empty the generator throw error instead of generating empty data.

[pantor]

Fixed with the latest commits.