Trap and report error when hostname is null/empty (uplift to 1.50.x)

[bsclifton]

Manual uplift of #17917
Fixes brave/brave-browser#28940
Fixes brave/brave-browser#29502
Fixes brave/brave-browser#29477

Pre-approval checklist:

• You have tested your change on Nightly.
• This contains text which needs to be translated.
  • There are more than 7 days before the release.
  • I've notified folks in #l10n on Slack that translations are needed.
• The PR milestones match the branch they are landing to.

Pre-merge checklist:

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.

Post-merge checklist:

• The associated issue milestone is set to the smallest version that the changes is landed on.

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[MadhaviSeelam]

Verification PASSED using

Brave | 1.50.113 Chromium: 112.0.5615.49 (Developer Build) (64-bit)
-- | --
Revision | bd2a7bcb881c11e8cfe3078709382934e3916914-refs/branch-heads/5615@{#936}
OS | Windows 11 Version 22H2 (Build 22621.1413)

• Installed 1.50.113
• launched Brave
• enabled brave://flags/#brave-vpn
• Logged into Brave VPN with Prod credentials
• Enabled VPN via VPN panel

Confirmed Brave no longer crashes and Failed to write config error is shown as expected

[kjozwiak]

Uplift into 1.50.x approved 👍 QA has verified that the crash doesn't occur anymore via 1.52.x as per #17917 (comment) and verified via 1.50.x using a PR build as per #17919 (comment).