Return nil in function_load if file is not present

The other file-opening functions might have similar behaviour, and still
need to be checked.

c/ton/main.c

		// TODO: should not load from here?
		snprintf(full_path, 150, "%s/%s", "out", path);
		JSValueRef contents_val = NULL;
		char *contents = get_contents(full_path);
		JSStringRef contents_str = JSStringCreateWithUTF8CString(contents);
		free(contents);
		if (contents != NULL) {
			JSStringRef contents_str = JSStringCreateWithUTF8CString(contents);
			free(contents);
			contents_val = JSValueMakeString(ctx, contents_str);
		}
		JSValueRef res[2];
		res[0] = JSValueMakeString(ctx, contents_str);
		res[0] = contents_val;
		res[1] = JSValueMakeNumber(ctx, 0);
		return JSObjectMakeArray(ctx, 2, res, NULL);
	}