fix #2550 inlay hints lambdas parameter type null reference exception #459

Job	Run time
Test (ubuntu-20.04, OmniSharp.MSBuild.Tests,OmniSharp.Roslyn.CSharp.Tests,OmniSharp.Cake.Tests,Om...	17m 32s
Test (ubuntu-20.04, OmniSharp.MSBuild.Tests,OmniSharp.Roslyn.CSharp.Tests,OmniSharp.Cake.Tests,Om...	17m 32s
Test (ubuntu-20.04, OmniSharp.DotNetTest.Tests)	8m 49s
Test (ubuntu-20.04, OmniSharp.DotNetTest.Tests)	8m 49s
Test (windows-2022, OmniSharp.MSBuild.Tests,OmniSharp.Roslyn.CSharp.Tests,OmniSharp.Cake.Tests,Om...	22m 28s
Test (windows-2022, OmniSharp.MSBuild.Tests,OmniSharp.Roslyn.CSharp.Tests,OmniSharp.Cake.Tests,Om...	22m 28s
Test (windows-2022, OmniSharp.DotNetTest.Tests)	12m 55s
Test (windows-2022, OmniSharp.DotNetTest.Tests)	12m 55s
Test (macos-11, OmniSharp.MSBuild.Tests,OmniSharp.Roslyn.CSharp.Tests,OmniSharp.Cake.Tests,OmniSh...	34m 40s
Test (macos-11, OmniSharp.MSBuild.Tests,OmniSharp.Roslyn.CSharp.Tests,OmniSharp.Cake.Tests,OmniSh...	29m 23s
Test (macos-11, OmniSharp.DotNetTest.Tests)	21m 0s
Test (macos-11, OmniSharp.DotNetTest.Tests)	21m 0s
	3h 49m 31s

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