fix cfngin diff output lookup resolution when no stack change

[ITProKyle]

Why This Is Needed

Outputs are not being set on the Stack object from Context if the CFN stack reports no change. This causes any output lookups to fail. Output lookups work fine when when the stack being referenced reports changes.

ERROR:runway.cfngin.plan:Couldn't resolve lookup in variable `TestParam`, lookup: ${Lookup<Literal<'output'> Concatenation[Literal<'fake-stack::FakeOutput'>]>}: (<class 'TypeError'>) 'NoneType' object is not subscriptable
Traceback (most recent call last):
  File "/Users/kyle/repos/runway/runway/variables.py", line 663, in resolve
    **kwargs)
  File "/Users/kyle/repos/runway/runway/cfngin/lookups/handlers/output.py", line 36, in handle
    return stack.outputs[decon.output_name]
TypeError: 'NoneType' object is not subscriptable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/kyle/repos/runway/runway/variables.py", line 675, in resolve
    provider=provider))
  File "/Users/kyle/repos/runway/runway/variables.py", line 710, in _resolve_legacy
    provider=provider)
  File "/Users/kyle/repos/runway/runway/cfngin/lookups/handlers/output.py", line 36, in handle
    return stack.outputs[decon.output_name]
TypeError: 'NoneType' object is not subscriptable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/kyle/repos/runway/runway/variables.py", line 103, in resolve
    variables=variables, **kwargs)
  File "/Users/kyle/repos/runway/runway/variables.py", line 677, in resolve
    raise FailedLookup(self, err2)
runway.cfngin.exceptions.FailedLookup: Failed lookup

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/kyle/repos/runway/runway/cfngin/plan.py", line 142, in _run_once
    status = self.fn(self.stack, status=self.status)
  File "/Users/kyle/repos/runway/runway/cfngin/actions/diff.py", line 177, in _diff_stack
    stack.resolve(self.context, provider)
  File "/Users/kyle/repos/runway/runway/cfngin/stack.py", line 231, in resolve
    resolve_variables(self.variables, context, provider)
  File "/Users/kyle/repos/runway/runway/variables.py", line 39, in resolve_variables
    variable.resolve(context=context, provider=provider)
  File "/Users/kyle/repos/runway/runway/variables.py", line 105, in resolve
    raise FailedVariableLookup(self.name, err.lookup, err.error)
runway.cfngin.exceptions.FailedVariableLookup: Couldn't resolve lookup in variable `TestParam`, lookup: ${Lookup<Literal<'output'> Concatenation[Literal<'fake-stack::FakeOutput'>]>}: (<class 'TypeError'>) 'NoneType' object is not subscriptable
INFO:runway.cfngin.ui:fake-stack-02: failed (Couldn't resolve lookup in variable `TestParam`, lookup: ${Lookup<Literal<'output'> Concatenation[Literal<'fake-stack::FakeOutput'>]>}: (<class 'TypeError'>) 'NoneType' object is not subscriptable)

What Changed

Fixed

• runway plan for cfngin modules will now properly resolve output lookups when the original stack did not change

Screenshots