Clear default value when input

[ZYSzys]

Fix #717

[codecov]

Codecov Report

Merging #719 into master will decrease coverage by 0.03%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
- Coverage   92.69%   92.66%   -0.04%     
==========================================
  Files          26       26              
  Lines        1041     1036       -5     
  Branches       19       19              
==========================================
- Hits          965      960       -5     
  Misses         76       76

Impacted Files	Coverage Δ
packages/inquirer/lib/prompts/input.js	97.5% <50%> (-0.28%)	⬇️

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[SBoudrias]

Thanks, this actually makes a lot of sense 😆