Speed up unused opens handling for empty results

[psfinaki]

Before:

BenchmarkDotNet=v0.13.2, OS=Windows 11 (10.0.22621.2861)
11th Gen Intel Core i7-1185G7 3.00GHz, 1 CPU, 8 logical and 4 physical cores
.NET SDK=8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2 DEBUG
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

Method	Mean	Error	StdDev	Median	Gen0	Allocated
UnusedOpens	924.6 us	41.93 us	119.0 us	889.5 us	11.7188	747.91 KB

After:

BenchmarkDotNet=v0.13.2, OS=Windows 11 (10.0.22621.2861)
11th Gen Intel Core i7-1185G7 3.00GHz, 1 CPU, 8 logical and 4 physical cores
.NET SDK=8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2 DEBUG
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

Method	Mean	Error	StdDev	Gen0	Allocated
UnusedOpens	80.31 us	1.602 us	2.348 us	0.6104	43.21 KB

[github-actions]

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/8.0.200.md