Fix: Cannot read property 'pass-thru' of undefined crash (#3984)

* Fix for #3908

Validate that value is available before reading configuration.

Offending plugin in my case with `imodeler1`

* Add changes

* Fix build error

* Apply suggestions from code review

Co-authored-by: Timothee Guerin <timothee.guerin@outlook.com>

* Update pipeline.ts

Co-authored-by: Timothee Guerin <timothee.guerin@outlook.com>

common/changes/@autorest/core/patch-1_2021-03-14-18-45.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "packageName": "@autorest/core",
+      "comment": "**Fix** Cannot read property 'pass-thru' of undefined crash",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@autorest/core",
+  "email": "kant2002@gmail.com"
+}

packages/extensions/core/src/lib/pipeline/pipeline.ts

@@ -359,12 +359,12 @@ export async function runPipeline(configView: AutorestContext, fileSystem: IFile
 
     // you can have --pass-thru:FOO on the command line
     // or add pass-thru: true in a pipline configuration step.
+    const configEntry = context.GetEntry(node.configScope.last.toString());
     const passthru =
-      context.GetEntry(node.configScope.last.toString())["pass-thru"] === true ||
+      configEntry?.["pass-thru"] === true ||
       values(configView.GetEntry("pass-thru")).any((each) => each === pluginName);
     const usenull =
-      context.GetEntry(node.configScope.last.toString())["null"] === true ||
-      values(configView.GetEntry("null")).any((each) => each === pluginName);
+      configEntry?.["null"] === true || values(configView.GetEntry("null")).any((each) => each === pluginName);
 
     const plugin = usenull ? plugins.null : passthru ? plugins.identity : plugins[pluginName];