Ensure Parameters property of arrow function is never nil

[ahejlsberg]

Fixes a crash in the language service:

const f = () => 42  // Delete () and LSP panics

[Copilot]

Pull Request Overview

This PR fixes a crash in the language service that occurs when parsing malformed arrow functions where the parameter list parentheses are deleted. The fix ensures that the Parameters property of arrow functions is never nil by providing an empty node list as a fallback.

• Adds initialization of parameters variable with an empty node list when parameter parsing fails in ambiguous parsing contexts

[jakebailey]

Hopefully all of the other signature like things all have this set too (it was required in the old code)

[ahejlsberg]

Hopefully all of the other signature like things all have this set too (it was required in the old code)

I check all the code paths for parsing signatures, and this should be the only one that could cause a nil.