Fix some incorrect parameter parsing in Gitlab Runner

[dovics]

Provide a description of what has been changed

1. Fix null pointer when runnerScopeFromEnv or ownerFromEnv environment variable is empty
2. Expose parsing errors when applicationID or installationID isn't int, and refine error reporting

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #6156 #6159

Relates to #