Fix: Filter undefined values when showing calls

When generating the message to display actually received calls on a
mock, we filtered out null values when joining our message lines.
Instead we should filter undefined as this is a possible return value of
`context.utils.diff`

Author: stschulte

Diff for: src/matcher.ts

@@ -7,7 +7,7 @@ import type { AwsCommand, AwsStub } from 'aws-sdk-client-mock';
 
 import { ObjectContaining } from '@vitest/expect';
 
-import { notNull, ordinalOf } from './utils.js';
+import { notUndefined, ordinalOf } from './utils.js';
 
 /**
  * We define some aliases
@@ -125,7 +125,7 @@ function formatCalls<Input extends object, Output extends MetadataBearer>(
                   .map(line => `    ${line}`)
                   .join('\n'),
             '',
-          ].filter(notNull);
+          ].filter(notUndefined);
         }),
         `Number of calls: ${calls.length.toString()}`,
       ].join('\n');

Diff for: src/utils.ts

@@ -2,6 +2,10 @@ export function notNull<T>(obj: null | T): obj is T {
   return obj !== null;
 }
 
+export function notUndefined<T>(obj: T | undefined): obj is T {
+  return obj !== undefined;
+}
+
 export function ordinalOf(n: number): string {
   const j = n % 10;
   const k = n % 100;

Diff for: tests/utils.test.ts

@@ -1,18 +1,32 @@
 import { describe, expect, it } from 'vitest';
 
-import { notNull, ordinalOf } from '../src/utils.js';
+import { notNull, notUndefined, ordinalOf } from '../src/utils.js';
 
 describe('notNull', () => {
   it('returns true when not null', () => {
-    expect(notNull('Hi')).toBe(true);
+    expect(notNull('Hi')).toBeTruthy();
   });
 
   it('returns false when null', () => {
-    expect(notNull(null)).toBe(false);
+    expect(notNull(null)).toBeFalsy();
   });
 
   it('returns true when undefined', () => {
-    expect(notNull(undefined)).toBe(true);
+    expect(notNull(undefined)).toBeTruthy();
+  });
+});
+
+describe('notUndefined', () => {
+  it('returns true when not undefined', () => {
+    expect(notUndefined('Hi')).toBeTruthy();
+  });
+
+  it('returns false when undefined', () => {
+    expect(notUndefined(undefined)).toBeFalsy();
+  });
+
+  it('returns true when null', () => {
+    expect(notUndefined(null)).toBeTruthy();
   });
 });