Fixes custom publisher tipping amounts

[ryanml]

Fixes: brave/brave-browser#2645

Turns out this already worked, just needed to change from amount -> amounts in the donate state enum.

Also updated the check to look for amounts.length, as if there are no custom amounts, we just get an empty list back.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

1. Enable Rewards
2. Navigate to https://kjozwiak.github.io/
3. Open panel, hit 'Send Tip' to bring up site banner
4. Ensure that the donation amount choices reflect the customizations made to the publisher banner

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[NejcZdovc]

master (0.61) 2bd242d
0.60 3a37a03
0.59 fa94258

[rebron]

Approving uplift request. cc: @kjozwiak @srirambv