Fix GetOutputBindings method to return correct output when OutputBindingData is not set

[kshyju]

Fixes #983

For GetOutputBindings method, currently the output binding entries are not returned when OutputBindingData dictionary does not have an entry for the binding item. This means, if the function execution was not successful (Ex: an exception being thrown from within the function code), the GetOutputBindings will return an empty collection.

With this fix, for the above mentioned use case, the GetOutputBindings method will return a collection of output binding entries with it's value property set to null which the user may overwrite with a non null value as needed.

Added a new test to validate this case.

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

[kshyju]

/check-enforcer evaluate