frontend-plugin-api: use undefined instead of null for extension definition .T

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/frontend-plugin-api/src/wiring/createExtension.ts

@@ -390,7 +390,7 @@ export function createExtension<
   return {
     $$type: '@backstage/ExtensionDefinition',
     version: 'v2',
-    T: null as unknown as T,
+    T: undefined as unknown as T,
     kind: options.kind,
     namespace: options.namespace,
     name: options.name,

packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts

@@ -23,7 +23,7 @@ import {
 describe('resolveExtensionDefinition', () => {
   const baseDef = {
     $$type: '@backstage/ExtensionDefinition',
-    T: null as any,
+    T: undefined as any,
     version: 'v2',
     attachTo: { id: '', input: '' },
     disabled: false,
@@ -65,7 +65,7 @@ describe('resolveExtensionDefinition', () => {
 describe('old resolveExtensionDefinition', () => {
   const baseDef = {
     $$type: '@backstage/ExtensionDefinition',
-    T: null as any,
+    T: undefined as any,
     version: 'v1',
     attachTo: { id: '', input: '' },
     disabled: false,