title: Modify Mono if other Mono is empty in project Reactor?
body: <p>I'm having a <code>Mono</code> that I want to modify when other <code>Mono</code> is empty.</p>
  
  <p>First <code>Mono</code>:</p>
  
  <pre><code>deviceRepository.getDevice(deviceId, userId): Mono&lt;Device&gt;
  </code></pre>
  
  <p>Second <code>Mono</code>:</p>
  
  <pre><code>registrationRepository.canRegister(deviceId) : Mono&lt;DeviceRegistrationInfo&gt;
  </code></pre>
  
  <p>What I want to achieve is return modified <code>Mono&lt;Device&gt;</code> when <code>canRegister</code> returns empty mono for a given <code>deviceId</code>: </p>
  
  <pre><code>device.copy(status = DeviceStatus.REGISTRATION_FAILED)
  </code></pre>
  
  <p>Similarly I want to modify each emission of <code>Flux&lt;Device&gt;</code> for combination of:</p>
  
  <pre><code>deviceRepository.getDevices(userId) : Flux&lt;Device&gt;
  </code></pre>
  
  <p>and </p>
  
  <pre><code>registrationRepository.getAllRegistrationInfos(): Flux&lt;DeviceRegistrationInfo&gt;
  </code></pre>