[dotnet] Unknowns for previews involving an uninitialized provider

[EronWright]

Proposed changes

This PR fixes the "panic" that occurs when an uninitialized provider is used with the yaml/kustomize resources. In that situation, the invoke calls aren't performed and the engine simply returns an uninitialized result. This causes a problem in the callers because the methods on ImmutableArray mustn't be used when the value is uninitialized (i.e. when IsDefault is true).

Example

The example given in #2741 easily repros the issue. With this fix, the behavior improves:

Diagnostics:
  kubernetes:yaml:ConfigFile (guestbook):
    warning: Required input properties have unknown values. Preview is incomplete.

Outputs:
    FrontendIp: output<string>

Testing

Includes integration tests involving an uninitialized provider with a ConfigFile or Directory.

Related issues (optional)

Closes #2741

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.18%. Comparing base (ee61f79) to head (7e15162).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2957   +/-   ##
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  Coverage   29.18%   29.18%           
=======================================
  Files          61       61           
  Lines        8196     8196           
=======================================
  Hits         2392     2392           
  Misses       5600     5600           
  Partials      204      204           

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[blampe]

The fix and regression test LGTM!

It's worth updating the changelog since this is user-facing.