Sign out fix

[powerful23]

Issue #, if available:
#510
Description of changes:
Users will get error when signing out if their identity pool doesn't allow unauthenticated access.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #511 into master will decrease coverage by 0.08%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master     #511      +/-   ##
==========================================
- Coverage   88.98%   88.89%   -0.09%     
==========================================
  Files          62       62              
  Lines        3059     3062       +3     
  Branches      591      591              
==========================================
  Hits         2722     2722              
- Misses        322      325       +3     
  Partials       15       15

Impacted Files	Coverage Δ
packages/aws-amplify/src/Auth/Auth.ts	85.81% <20%> (-0.47%)	⬇️

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[github-actions]

This pull request has been automatically locked since there hasn't been any recent activity after it was closed. Please open a new issue for related bugs.

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