fix(internal/secrets): Error on Missing Secrets

[jshlbrd]

Description

• Fixes a missing error in case no secret is found

Motivation and Context

Discovered this after running substation vet -R examples/:

% substation vet examples/transform/enrich/urlscan/config.jsonnet
panic: interface conversion: interface {} is nil, not string

This is the new output:

% substation vet examples/transform/enrich/urlscan/config.jsonnet
examples/transform/enrich/urlscan/config.jsonnet:2 transform bb7942d9-810481d2: secrets: no secret found

    {"type":"enrich_http_post","settings":{"headers":{"API-Key":"${SECRET:URLSCAN}","Content-Type":"application/json"},"id":"bb7942d9-810481d2","object":{"body_key":"@this","target_key":"meta response"},"request":{"timeout":"1s"},"url":"https://urlscan.io/api/v1/scan/"}}

How Has This Been Tested?

Using substation vet (v2.1.0).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.