[FEATURE] Do not crash the whole script if there's no config entry

[Panquesito7]

Detailed description

The latest release added an option to customize the PR labels. This can be added by modifying the repos.lua file.
However, if there's no config entry provided, the script will attempt to get information from an entry that doesn't exist, which will result in crashes.

Context

This will prevent errors when the user doesn't have a config entry provided, which will also help in using default values.

Possible implementation

Instead, provide default PR labels if they're not available or manually set.

Additional information

No response

Code of Conduct

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