Rewards error logged repeatedly on private new tab page

[zenparsing]

Description

In a private browser context, the new tab page will repeatedly log the following error:

"Unchecked runtime.lastError: Rewards service is not initialized", source: chrome://newtab/ (0)

Steps to Reproduce

1. Open a new private window.
2. Open developer tools and select the console tab.
3. Wait 30 seconds.

Actual result:

Every 30 seconds a rewards error is logged.

Expected result:

An error should not be logged.

[stephendonner]

Verified PASSED using nightly build

Brave	1.25.46 Chromium: 90.0.4430.93 (Official Build) nightly (x86_64)
Revision	4df112c29cfe9a2c69b14195c0275faed4e997a7-refs/branch-heads/4430@{#1348}
OS	macOS Version 11.3 (Build 20E232)

Confirmed the Rewards error from #15417 (comment) is no longer spamming the console's Error tab.