Fix error handling in LSP to guard against empty results from the compiler.

[JoshuaBatty]

Description

Fixes the crashes that we were seeing in CI. We now return an error if the last results is empty so we don't index oob when accessing the Engines during traversal or when creating runnables.

closes #6458

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[github-actions]

Benchmark for e063831

Click to view benchmark

Test	Base	PR	%
code_action	5.0±0.09ms	5.0±0.01ms	0.00%
code_lens	283.2±5.53ns	293.3±34.94ns	+3.57%
compile	1882.8±55.97ms	1910.4±54.14ms	+1.47%
completion	4.4±0.09ms	4.4±0.03ms	0.00%
did_change_with_caching	1791.0±22.22ms	1826.8±29.51ms	+2.00%
document_symbol	866.7±8.58µs	903.8±17.71µs	+4.28%
format	83.0±0.99ms	83.2±1.19ms	+0.24%
goto_definition	336.6±3.57µs	346.3±5.48µs	+2.88%
highlight	8.7±0.12ms	8.9±0.75ms	+2.30%
hover	345.0±4.97µs	355.3±6.52µs	+2.99%
idents_at_position	117.2±0.38µs	118.2±0.97µs	+0.85%
inlay_hints	640.6±25.74µs	630.8±11.23µs	-1.53%
on_enter	1936.0±38.25ns	2.1±0.13µs	+8.47%
parent_decl_at_position	3.5±0.03ms	3.6±0.02ms	+2.86%
prepare_rename	335.0±7.32µs	345.8±6.15µs	+3.22%
rename	9.0±0.20ms	9.0±0.01ms	0.00%
semantic_tokens	1182.1±20.74µs	1211.9±12.78µs	+2.52%
token_at_position	331.4±2.94µs	331.3±4.43µs	-0.03%
tokens_at_position	3.5±0.04ms	3.6±0.03ms	+2.86%
tokens_for_file	400.0±2.00µs	394.9±1.93µs	-1.28%
traverse	33.7±0.55ms	34.0±0.71ms	+0.89%