Avoid updating scans schedule on helm upgrades when not explicitly set

[matheusfm]

Description

These changes avoid updates scans schedule on helm upgrades, when schedules are not set.

How has this been tested?

These changes can be tested with helm template --dry-run=server or helm upgrade --install commands in clusters without Zora installed and clusters with Zora already installed.

• For clusters without Zora installed, the behavior should keep the same: if a scan schedule is explicitly set, use it; otherwise use a default value with 5 minutes added.
• For clusters with Zora already installed, the schedules should not be updated unless it's explicitly set.

Checklist

• I have labeled this PR with the relevant Type labels
• I have documented my code (if applicable)
• My changes are covered by tests