Fix parameter_assignments documentation for null safety and updated dart syntax

[IldySilva]

Fix 7048

• Fixed incorrect use of ??= on non-nullable parameters
• Converted affected parameters to nullable where appropriate (int → int?)
• Ensured each BAD example triggers exactly one diagnostic (the parameter_assignments lint)
• Ensured each GOOD example produces zero diagnostics
• Removed logically dead code caused by non-nullable ??= usage