Signature help crashes at any location with no signature

[DanielRosenwasser]

Probably related to all the other signature help crashes, but worth adding a test if it's not caught by fourslash.

let x = /**/

Steps to reproduce

1. Request signature help
2. Crash

panic handling request textDocument/signatureHelp runtime error: invalid memory address or nil pointer dereference goroutine 1512 [running]:
runtime/debug.Stack()
	/home/daniel/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25rc1.linux-arm64/src/runtime/debug/stack.go:26 +0x64
github.com/microsoft/typescript-go/internal/lsp.(*Server).dispatchLoop.func1.1()
	/home/daniel/typescript-go/internal/lsp/server.go:357 +0x50
panic({0x5ee320?, 0xe2de00?})
	/home/daniel/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25rc1.linux-arm64/src/runtime/panic.go:783 +0x120
github.com/microsoft/typescript-go/internal/ls.(*LanguageService).GetSignatureHelpItems(0x4001f40210?, {0x8f3dd0?, 0x4006111770?}, 0x8, 0x7a2021?, 0x40075f0848, 0x0, 0x40002aacf0, 0x4005dbf880?)
	/home/daniel/typescript-go/internal/ls/signaturehelp.go:77 +0x88
github.com/microsoft/typescript-go/internal/ls.(*LanguageService).ProvideSignatureHelp(0x4007ed7bf0, {0x8f3dd0, 0x4006111770}, {0x4002f154d0?, 0x400249cb40?}, {0x6bb320?, 0x0?}, 0x0, 0x40002aacf0, 0x4004b69e70)
	/home/daniel/typescript-go/internal/ls/signaturehelp.go:47 +0x98
github.com/microsoft/typescript-go/internal/lsp.(*Server).handleSignatureHelp(0x400017ea20, {0x8f3dd0, 0x4006111770}, 0x4008332810)
	/home/daniel/typescript-go/internal/lsp/server.go:684 +0xd4
github.com/microsoft/typescript-go/internal/lsp.(*Server).handleRequestOrNotification(0x4003065f68?, {0x8f3dd0?, 0x4006111770?}, 0x48dc30?)
	/home/daniel/typescript-go/internal/lsp/server.go:494 +0x94
github.com/microsoft/typescript-go/internal/lsp.(*Server).dispatchLoop.func1()
	/home/daniel/typescript-go/internal/lsp/server.go:370 +0x60
created by github.com/microsoft/typescript-go/internal/lsp.(*Server).dispatchLoop in goroutine 19
	/home/daniel/typescript-go/internal/lsp/server.go:388 +0x448

[DanielRosenwasser]

It's now happening even at places with signatures.

[jakebailey]

If the stack is instead about JSON, I did just fix a bunch of that on main.

[jakebailey]

I suspect #1469 fixed this (in VS Code), though @navya9singh is working on a better fix that nil checks the context since it's technically optional in LSP.