Fix signature help crash for type arguments when call target is unresolved

[DanielRosenwasser]

Fixes #2294.

[Copilot]

Pull request overview

This PR fixes a crash in signature help when type arguments are provided for an unresolved call target. The fix adds proper nil checking before returning symbol information, preventing a nil pointer dereference in downstream code.

Key changes:

• Added nil check for GetSymbolAtLocation result in the type arguments invocation path
• Return nil when the symbol cannot be resolved instead of returning a struct with a nil typeInfo field
• Added two test cases to verify the crash is fixed and behavior is correct

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/ls/signaturehelp.go	Added nil check for unresolved symbols in type arguments invocation path and improved code readability with blank lines
internal/fourslash/tests/signatureHelpTypeArgumentsWithUntypedTarget_test.go	Added test to verify no signature help is shown for unresolved identifiers with type arguments
internal/fourslash/tests/signatureHelpBetweenTypeArgsAndArgs_test.go	Added test to verify no crash when signature help is requested between type arguments and regular arguments