[Canvas] Fixes Error 500 on the workpad update. (#111683)

* Fixed bug with treating null argument as the expression function.

* Added a test for checking if a null argument is processeded correctly.

Co-authored-by: Kibana Machine <[email protected]>

src/plugins/expressions/common/executor/executor.test.ts

@@ -197,6 +197,11 @@ describe('Executor', () => {
           types: ['string'],
           help: 'other arg',
         },
+        nullable: {
+          types: ['string', 'null'],
+          help: 'nullable arg',
+          default: null,
+        },
       },
       extract: (state: ExpressionAstFunction['arguments']) => {
         const references: SavedObjectReference[] = [
@@ -254,6 +259,14 @@ describe('Executor', () => {
 
         expect(formatExpression(executor.inject(state, references))).toBe(expression);
       });
+
+      test('allows expression function argument to be null', () => {
+        const expression = `ref nullable=null id="my-id" other={ref id="nested-id" other="other" | foo bar="baz"}`;
+        const { state, references } = executor.extract(parseExpression(expression));
+
+        expect(state.chain[0].arguments.nullable[0]).toBeNull();
+        expect(formatExpression(executor.inject(state, references))).toBe(expression);
+      });
     });
 
     describe('.getAllMigrations', () => {

src/plugins/expressions/common/executor/executor.ts

@@ -204,7 +204,7 @@ export class Executor<Context extends Record<string, unknown> = Record<string, u
         // if any of arguments are expressions we should migrate those first
         link.arguments = mapValues(fnArgs, (asts, argName) => {
           return asts.map((arg) => {
-            if (typeof arg === 'object') {
+            if (arg && typeof arg === 'object') {
               return this.walkAst(arg, action);
             }
             return arg;