strict null check 'vs/workbench/parts/terminal/electron-browser/'

[mjbvz]

Part of #60565

Tracks updating ./vs/workbench/parts/terminal/electron-browser/ files to have strict null checking. The following files are currently eligible:

• vs/workbench/parts/terminal/electron-browser/terminalInstance.ts
• vs/workbench/parts/terminal/electron-browser/terminalLinkHandler.ts
• vs/workbench/parts/terminal/electron-browser/terminalProcessManager.ts

Steps

1. Add file to the files section of src/tsconfig.strictNullChecks.json
2. Run yarn strict-null-check -- --watch
3. Fix strict null check compiler errors
4. Verify normal build still works