fix(dependency_getter): avoid error on empty dependencies

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added or modified a feature, documentation in docs is updated

Description of changes

We currently hard access dependencies key for both PEP 621 and Poetry projects to retrieve dependencies. Although not common, it could happen that projects use deptry, but do not have any production dependencies (for instance when bootstrapping a project from a minimal template, where you don't start with any dependency, but still want to include deptry in the CI workflow).

This PR soft accesses dependencies for both PEP 621 and Poetry. As for Poetry, we rely on dependencies to determine if a project uses Poetry, the logic is also updated to assume that a project uses by only checking [tool.poetry] instead of [tool.poetry.dependencies]. This should not be problematic, as [tool.poetry] is specific to Poetry, and Poetry only.

[codecov]

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Additional details and impacted files

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  Hits            ?     856           
  Misses          ?      50           
  Partials        ?      14           

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