fix: adjust config flow for amazon forwards

[firstof9]

Proposed change

Adjust the amazon forwards config flow to hopefully allow empty strings.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests
• Documentation update
• Adds a new shipper
• Update existing shipper

Additional information

• This PR is related to issue: Configuration Error - Amazon Forwarded Email Addresses #689

[codecov-commenter]

Codecov Report

Merging #693 (28e65ad) into dev (c875602) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #693   +/-   ##
=======================================
  Coverage   98.15%   98.15%           
=======================================
  Files           8        8           
  Lines        1462     1462           
=======================================
  Hits         1435     1435           
  Misses         27       27           

Impacted Files	Coverage Δ
custom_components/mail_and_packages/config_flow.py	100.00% <ø> (ø)