fix: ignore invalid integration keys when building entity reference dict

[brianphillips]

Description

When building the mapping between service ID and backstage entities, any invalid data in the catalog would result in an error (as reported in #68). This PR fixes #68.

Issue number: #68

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [x] Fix (non-breaking change which fixes an issue)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [x] I have performed a self-review of this change
• [x] Changes have been tested
• [ ] Changes are documented
• [x] Changes generate no new warnings
• [x] PR title follows conventional commit semantics

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