Exit email attachment iteration unless image with content_id is present

[andrewmcodes]

Type of PR

• [ ] feature
• [ ] enhancement
• [x] bug fix
• [ ] other

Description

Stop calling replace on nil when document.at_css "img[src='cid:#{content_id}']" is not found while iterating through incoming email attachments

Fixes #1316

Checklist

• [x] My code follows the style guidelines of this project & the tests are passing
• [x] I have performed a self-review of my own code
• [x] I have requested a review from another developer(s)

Screenshots

[codecov[bot]]

Codecov Report

Merging #1317 into master will decrease coverage by 0.00%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1317      +/-   ##
==========================================
- Coverage   34.76%   34.75%   -0.01%     
==========================================
  Files         345      345              
  Lines        7989     7990       +1     
==========================================
  Hits         2777     2777              
- Misses       5212     5213       +1     

Impacted Files	Coverage Δ
app/mailboxes/incoming_mailbox.rb	64.28% <0.00%> (-1.57%)	:arrow_down: