fix: Fix inputs parsing when ctx.inputs is undefined.

[batk0]

A tiny fix for parsing inputs. It's for the case when spec.inputs are specified in main .gitlab-ci.yml with defaults and no inputs provided for the job.

Example:

---
spec:
  inputs:
    foo:
      type: string
      default: bar
---
job:
  script:
    - echo $[[ inputs.foo ]]

[ANGkeith]

@batk0 could you fix the failing test

other than that lgtm

[batk0]

@ANGkeith Done