Check if sync enabled before calling (crash fix)

[bsclifton]

Fixes brave/brave-browser#25156

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Fresh profile; launch Brave
2. Set brave://flags/#brave-sync-v2 to Disabled
3. Relaunch
4. Open settings (brave://settings)
5. Tab should load (no crash)

[mkarolin]

++

[kjozwiak]

Reproduced the original crash on Win 11 x64 using the following build(s):

Brave | 1.45.34 Chromium: 105.0.5195.102 (Official Build) nightly (64-bit)
-- | --
Revision | 4c16f5ffcc2da70ee2600d5db77bed423ac03a5a-refs/branch-heads/5195_55@{#4}
OS | Windows 11 Version 21H2 (Build 22000.918)

Went through the STR/Cases outlined via #15031 (comment) and managed to reproduce the crash when loading brave://settings once brave://flags/#brave-sync-v2 has been disabled.

Verification PASSED on Win 11 x64 using the following build(s):

Brave | 1.45.35 Chromium: 105.0.5195.102 (Official Build) nightly (64-bit)
-- | --
Revision | 4c16f5ffcc2da70ee2600d5db77bed423ac03a5a-refs/branch-heads/5195_55@{#4}
OS | Windows 11 Version 21H2 (Build 22000.918)

Went through the STR/Cases outlined via #15031 (comment) ensured that disabling brave://flags/#brave-sync-v2 and visiting brave://settings didn't crash Brave. Also ensured that Sync was removed from the Hamburger Menu and there were no issues opening/closing the menu.