updatesAvailable not always available

The reading `updatesAvailable` is not always available. If no packages are outdated, the reading is not created. Should be always present, but filled with `0` if no package updates are available.

```
Internals:
.FhemMetaInternals 1
.eventMapCmd Aktualisieren:noArg
CFGFN ./cfg.d/general/network/hosts/hab.cfg
DEF localhost
FUUID 6037825d-f33f-a67d-f0d7-0844bcadabb2d622
FVERSION 42_AptToDate.pm:v1.4.5-s19639/2019-06-18
HOST localhost
NAME general.network.hosts.hab0.packet_manager
NOTIFYDEV global,general.network.hosts.hab0.packet_manager
NR 4149
NTFY_ORDER 50-general.network.hosts.hab0.packet_manager
STATE [general.network.hosts.hab0.packet_manager:updatesAvailable] Updates verfügbar
TYPE AptToDate
VERSION v1.4.5
.attraggr:
.attrminint:
.fhem:
aptget:
cmd repoSync
debug 0
distupgrade 0
host localhost
lang de
packages:
warnings:
HASH(0xd1f1808)
HASH(0xbf8f648)
Helper:
DBLOG:
Aktualisieren:
general.system.log.db:
TIME 1614707867.91424
VALUE fetched warnings
state:
general.system.log.db:
TIME 1614707867.91424
VALUE warnings
READINGS:
2020-12-28 12:21:16 os-release_BUG_REPORT_URL http://www.raspbian.org/RaspbianBugs
2020-12-28 12:21:16 os-release_HOME_URL http://www.raspbian.org/
2020-12-28 12:21:16 os-release_ID raspbian
2020-12-28 12:21:16 os-release_ID_LIKE debian
2020-12-28 12:21:16 os-release_NAME Raspbian GNU/Linux
2020-12-28 12:21:16 os-release_PRETTY_NAME Raspbian GNU/Linux 10 (buster)
2020-12-28 12:21:16 os-release_SUPPORT_URL http://www.raspbian.org/RaspbianForums
2020-12-28 12:21:16 os-release_VERSION 10 (buster)
2020-12-28 12:21:16 os-release_VERSION_CODENAME buster
2020-12-28 12:21:16 os-release_VERSION_ID 10
2020-12-28 12:21:16 os-release_language de
2021-03-02 18:57:47 repoSync fetched warnings
2021-03-02 18:57:47 state warnings
helper:
lastSync 2021-03-02
Attributes:
alias Paketmanager-Status
devStateIcon system.updates.available:security@red system.is.up.to.date:security@green:repoSync .*in.progress:system_fhem_reboot@orange errors:message_attention@red
eventMap repoSync:Aktualisieren
group hab0.home.net
icon debian@black
room Admin->Netzwerk
sortby 100
stateFormat [$name:updatesAvailable] Updates verfügbar
upgradeListReading 1
webCmd Aktualisieren