fix: setPreferredMFA method throws error when setting totp with no cellphone

[powerful23]

Issue #, if available:
fixes #806 #618 #846
Description of changes:

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 #859 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #859   +/-   ##
======================================
  Coverage    87.3%   87.3%           
======================================
  Files          77      77           
  Lines        3670    3670           
  Branches      696     696           
======================================
  Hits         3204    3204           
  Misses        443     443           
  Partials       23      23

Impacted Files	Coverage Δ
packages/aws-amplify/src/Auth/Auth.ts	88.03% <100%> (ø)	⬆️

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[mlabieniec]

👍

[github-actions]

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