fix(smarty): fix php warning in poller configuration form

[jeremyjaouen]

Description

This PR intends to fix php warning in poller configuration form due to a wrong way to check variable existence.

Fixes # MON-15471

Type of change

• [x] 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

Target serie

• [ ] 21.04.x
• [ ] 21.10.x
• [ ] 22.04.x
• [x] 22.10.x (master)

How this pull request can be tested ?

In Configuration > Pollers:

• Click on your Central
• When the update form displays, no php warning should occurs.

Checklist

Community contributors & Centreon team

• [ ] 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.
• [x] I have rebased my development branch on the base branch (master, maintenance).

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