Strict null pass on some base ui files (#4832)

* more strict null checks in base browser code

* revert changes to radiobutton

* fix some more minor things, enable strict null check in pipelines

* formatting

* fix compile errors

* make null undefined

* more null to undefined

azure-pipelines-linux-mac.yml

@@ -41,5 +41,9 @@ steps:
   condition: succeededOrFailed()
 
 - script: |
-    yarn run tslint
+    yarn tslint
   displayName: 'Run TSLint'
+
+- script: |
+    yarn strict-null-check
+  displayName: 'Run Strict Null Check'