Make the ZoomIt AAC audio track depend on system audio or audio input (#45700)

When neither option is selected, then the resulting .mp4 container won't
have an audio track added to it.
This is due to the recent audio input addition. As a result, all .mp4
recordings now have an audio track, even if empty. Restoring the prior
functionality, where just the h264 video is present on no audio.

## Summary of the Pull Request

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## PR Checklist

- [ ] Closes: #xxx
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issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
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pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
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- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/ZoomIt/ZoomIt/VideoRecordingSession.cpp

@@ -940,10 +940,12 @@ VideoRecordingSession::VideoRecordingSession(
     video.PixelAspectRatio().Denominator(1);
     m_encodingProfile.Video(video);
 
-    // Always set up audio profile for loopback capture (stereo AAC)
-    auto audio = m_encodingProfile.Audio();
-    audio = winrt::AudioEncodingProperties::CreateAac(48000, 2, 192000);
-    m_encodingProfile.Audio(audio);
+    if (captureAudio || captureSystemAudio)
+    {
+        auto audio = m_encodingProfile.Audio();
+        audio = winrt::AudioEncodingProperties::CreateAac(48000, 2, 192000);
+        m_encodingProfile.Audio(audio);
+    }
 
     // Describe our input: uncompressed BGRA8 buffers
     auto properties = winrt::VideoEncodingProperties::CreateUncompressed(
@@ -964,8 +966,14 @@ VideoRecordingSession::VideoRecordingSession(
     winrt::check_hresult(m_previewSwapChain->GetBuffer(0, winrt::guid_of<ID3D11Texture2D>(), backBuffer.put_void()));
     winrt::check_hresult(m_d3dDevice->CreateRenderTargetView(backBuffer.get(), nullptr, m_renderTargetView.put()));
 
-    // Always create audio generator for loopback capture; captureAudio controls microphone
-    m_audioGenerator = std::make_unique<AudioSampleGenerator>(captureAudio, captureSystemAudio, micMonoMix);
+    if (captureAudio || captureSystemAudio)
+    {
+        m_audioGenerator = std::make_unique<AudioSampleGenerator>(captureAudio, captureSystemAudio, micMonoMix);
+    }
+    else
+    {
+        m_audioGenerator = nullptr;
+    }
 }
 
 
@@ -1207,14 +1215,8 @@ void VideoRecordingSession::OnMediaStreamSourceSampleRequested(
     {
         try
         {
-            if (auto sample = m_audioGenerator->TryGetNextSample())
-            {
-                request.Sample(sample.value());
-            }
-            else
-            {
-                request.Sample(nullptr);
-            }
+            auto sample = m_audioGenerator ? m_audioGenerator->TryGetNextSample() : std::optional<winrt::MediaStreamSample>{};
+            request.Sample(sample.has_value() ? sample.value() : nullptr);
         }
         catch (winrt::hresult_error const& error)
         {