Return nil in function_read_file if not found

As with `function_load`, this must return nil for the rest to work.
(Otherwise noone can detect if the file is not present or empty.)

c/ton/main.c

		snprintf(full_path, 150, "%s/%s", "out", path);
		char *contents = get_contents(full_path);
		JSStringRef contents_str = JSStringCreateWithUTF8CString(contents);
		free(contents);
		if (contents != NULL) {
			JSStringRef contents_str = JSStringCreateWithUTF8CString(contents);
			free(contents);
		JSValueRef res[2];
		res[0] = JSValueMakeString(ctx, contents_str);
		res[1] = JSValueMakeNumber(ctx, 0);
		return JSObjectMakeArray(ctx, 2, res, NULL);
			JSValueRef res[2];
			res[0] = JSValueMakeString(ctx, contents_str);
			res[1] = JSValueMakeNumber(ctx, 0);
			return JSObjectMakeArray(ctx, 2, res, NULL);
		}
	}
	return JSValueMakeNull(ctx);