Stop MailJet from erroring when there are no emails to send

[mlt47]

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Pull Request Check List

• Unit Tests & Regression Tests Added (Optional)
• Removed Unnecessary Logs/System.Outs/Comments/TODOs
• Added enough Logging to monitor expected behaviour change
• Security - Injection - everything run by an interpreter (SQL, OS...) is either validated or escaped
• Security - Data Exposure - PII is not stored or sent unencrypted
• Security - Data Exposure - Test any altered or created endpoints using swagger
• Updated Release Procedure & Documentation (& Considered Implications to Previous Versions)
• Peer-Reviewed

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: +0.11% 🎉

Comparison is base (990e552) 27.80% compared to head (21eb833) 27.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
+ Coverage   27.80%   27.92%   +0.11%     
==========================================
  Files         504      504              
  Lines       22949    22954       +5     
  Branches     2842     2844       +2     
==========================================
+ Hits         6382     6410      +28     
+ Misses      15961    15932      -29     
- Partials      606      612       +6     

Files Changed	Coverage Δ
.../ac/cam/cl/dtg/segue/comm/MailGunEmailManager.java	49.29% <33.33%> (+38.68%)	⬆️

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