Do not throw exception when DependencyModel does not exist

src/Microsoft.Extensions.DependencyModel/CompilationLibrary.cs

@@ -28,7 +28,6 @@ namespace Microsoft.Extensions.DependencyModel
         public IEnumerable<string> ResolveReferencePaths()
         {
             var entryAssembly = _entryAssembly.Value;
-            var entryAssemblyName = entryAssembly.GetName().Name;
 
             string basePath;
             string fullName;

src/Microsoft.Extensions.DependencyModel/DependencyContext.cs

@@ -42,7 +42,7 @@ namespace Microsoft.Extensions.DependencyModel
 
             if (stream == null)
             {
-                throw new InvalidOperationException("Entry assembly was compiled without `preserveCompilationContext` enabled");
+                return null;
             }
 
             using (stream)