fix(policies): fail on required arguments that are empty after applying interpolations

[jiparis]

This PR fixes the case when a required argument gets an interpolated value, and the interpolation variables are not passed. In this case, if it renders to an empty string, should be still considered missing, thus failing the validation.

This one should fail if foo value is not provided and policyfoo is required by the policy.

apiVersion: workflowcontract.chainloop.dev/v1
kind: PolicyGroup
metadata:
  name: my-group
spec:
  inputs:
    - name: foo
policies:
  attestation:
    - ref: my-policy
      with: 
        policyfoo: "{{ inputs.foo }}"

Fixes #2012