Fix Brave Ads rewards migration path

[tmancey]

Description

Brave Ads rewards migration path fails if ads_rewards, transaction_history or unblinded_payment_tokens nodes are missing from Default/confirmations.json when migrating

Steps to Reproduce

1. If ads_rewards, transaction_history or unblinded_payment_tokens nodes are missing from Default/confirmations.json

Actual result:

Failed to initialize ads is shown in the verbose rewards logs.

Expected result:

Successfully initialized ads should be shown in the verbose rewards logs.

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

[btlechowski]

Verification passed on

Brave	1.45.75 Chromium: 106.0.5249.65 (Official Build) beta (64-bit)
Revision	3269dc3633cdd2ab94546fdbe54962e45b17a6e0-refs/branch-heads/5249@{#580}
OS	Ubuntu 18.04 LTS

Verified test plan from the description