fix: disable duplicate checks when threshold <= 0

[43081j]

If the threshold is 0, you're basically saying "show me when a package
has 0 or more versions". This will always be true so is a nonsensical
value.

Similarly, -1 may be set as a way to turn it off but currently does
the opposite (same as 0).

This change means -1 and 0 behave as "turn off duplicate checks".

[github-actions]

⚠️ Duplicate Dependencies (found: 3, threshold: 1)

📦 Package	📋 Versions
@actions/http-client	2 versions @e18e/action-dependency-diff@0.0.1 @actions/github@9.1.1 @actions/http-client@3.0.2 @e18e/action-dependency-diff@0.0.1 @actions/core@3.0.1 @actions/http-client@4.0.0
eslint-visitor-keys	2 versions @e18e/action-dependency-diff@0.0.1 eslint@10.5.0 @eslint-community/eslint-utils@4.9.1 eslint-visitor-keys@3.4.3 @e18e/action-dependency-diff@0.0.1 eslint@10.5.0 eslint-visitor-keys@5.0.1
ignore	2 versions @e18e/action-dependency-diff@0.0.1 eslint@10.5.0 ignore@5.3.2 @e18e/action-dependency-diff@0.0.1 typescript-eslint@8.62.0 @typescript-eslint/eslint-plugin@8.62.0 ignore@7.0.5

💡 To find out what depends on a specific package, run: npm ls example-package