fix(auth): handle Empty String Tax Display Name

[IvoJP]

Because:

• stripe started returning an empty string for tax display name which causes our response validation to fail

This commit:

• ORs an empty string with undefined so that undefined is returned when display name is an empty string

Closes #FXA-7811

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).