fix(downtime): fix error for downtime on hostgroups

[adr-mo]

This issue only occurs on 21.10, this is visible now thanks to the php upgrade.

• 21.04 runs with PHP 7.3, a count on a "null" variable will throw a warning message
• 21.10 runs with PHP 8.0, a count on a "null" variable will throw a fatal error

Fixes # (issue)

Type of change

• Patch fixing an issue (non-breaking change)
• New functionality (non-breaking change)
• Breaking change (patch or feature) that might cause side effects breaking part of the Software
• Updating documentation (missing information, typo...)

Target serie

• 20.04.x
• 20.10.x
• 21.04.x
• 21.10.x (master)

How this pull request can be tested ?

See Jira ticket for details

Checklist

• I have followed the coding style guidelines provided by Centreon
• I have commented my code, especially new classes, functions or any legacy code modified. (docblock)
• I have commented my code, especially hard-to-understand areas of the PR.
• I have made corresponding changes to the documentation.
• I have rebased my development branch on the base branch (master, maintenance).

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SonarQube Quality Gate:

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication