Add null check before calling Path.GetFullPath

[wscho77]

filePath can be null when AddModule() is called by InheritOpenModules().

System.ArgumentNullException is occured when crossgen2 run with "--out-near-input --single-file-compilation" options.
To fix this bugs, add null check code in AddModule()

root:/usr/share/dotnet/shared/Microsoft.NETCore.App/7.0.0> ./corerun ./crossgen2/crossgen2.dll --jitpath /usr/share/dotnet/shared/Microsoft.NETCore.App/7.0.0/libclrjit.so --targetarch arm64 -r:/usr/share/dotnet/shared/Microsoft.NETCore.App/7.0.0/*.dll --out-near-input --single-file-compilation /home/owner/HelloConsole.dll 
Error: Value cannot be null. (Parameter 'path')
System.ArgumentNullException: Value cannot be null. (Parameter 'path')
   at System.ArgumentNullException.Throw(String paramName)
   at System.ArgumentNullException.ThrowIfNull(Object argument, String paramName)
   at System.ArgumentException.ThrowNullOrEmptyException(String argument, String paramName)
   at System.IO.Path.GetFullPath(String path)
   at ILCompiler.CompilerTypeSystemContext.AddModule(String filePath, String expectedSimpleName, Boolean useForBinding, ModuleData oldModuleData, Boolean throwOnFailureToLoad) in /runtime/src/coreclr/tools/Common/Compiler/CompilerTypeSystemContext.cs:line 187
   at ILCompiler.CompilerTypeSystemContext.InheritOpenModules(CompilerTypeSystemContext oldTypeSystemContext) in /runtime/src/coreclr/tools/Common/Compiler/CompilerTypeSystemContext.cs:line 266
   at ILCompiler.ReadyToRunCompilerContext..ctor(TargetDetails details, SharedGenericsMode genericsMode, Boolean bubbleIncludesCorelib, CompilerTypeSystemContext oldTypeSystemContext) in /runtime/src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/ReadyToRunCompilerContext.cs:line 64
   at ILCompiler.Program.Run() in /runtime/src/coreclr/tools/aot/crossgen2/Program.cs:line 265
   at ILCompiler.Crossgen2RootCommand.<>c__DisplayClass181_0.<.ctor>b__0(InvocationContext context) in /runtime/src/coreclr/tools/aot/crossgen2/Crossgen2RootCommand.cs:line 287

[MichalStrehovsky]

Thanks!

[danmoseley]

ready to merge @MichalStrehovsky ?