Exception in cuebot if service icon is not present

**Describe the bug**
If a layer is running using a custom service that does not have an icon on the cuebot server, it gets en exception that it's not able to read the image.

**To Reproduce**
Steps to reproduce the behavior:
1. Submit job with eg. the `blender` service
2. Wait for job to complete
3. See error in cuebot log

**Expected behavior**
Icons for custom services should be optional and not required. It should not bring an exception

**Error Log**
```java
2024-07-22 21:43:15,990 ERROR pool-3-thread-1 com.imageworks.spcue.service.EmailSupport - Unable to read services/blender.png
java.io.IOException: Unable to load
at com.imageworks.spcue.service.EmailSupport.loadImage(EmailSupport.java:101) [main/:?]
at com.imageworks.spcue.service.EmailSupport.sendShutdownEmail(EmailSupport.java:263) [main/:?]
at com.imageworks.spcue.service.JobManagerSupport.shutdownJob(JobManagerSupport.java:160) [main/:?]
at com.imageworks.spcue.dispatcher.commands.DispatchJobComplete$1.wrapDispatchCommand(DispatchJobComplete.java:49) [main/:?]
at com.imageworks.spcue.dispatcher.commands.DispatchCommandTemplate.execute(DispatchCommandTemplate.java:36) [main/:?]
at com.imageworks.spcue.dispatcher.commands.DispatchJobComplete.run(DispatchJobComplete.java:51) [main/:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) [?:?]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) [?:?]
at java.lang.Thread.run(Thread.java:830) [?:?]
```

**Version Number**
688425ce859f183126ab8235543e4b0a13ff6c50

**Additional context**
Seems related to the new email template when jobs are finished.