Preserve new line on exchanges (x) with empty hold spaces.

[AlexanderWangY]

Fixes #254

• On exchange where hold space is empty, we ignore exchanging the newline.
• Only when the hold space is filled or the hold space has a newline do we exchange the newline
• Added test case

Note: It got me thinking, will there ever be a case where we exchange a line and have the new space be part of the line itself? It seems as if we won't really have to set it.

[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 64 / PASSED: 8 / FAILED: 56 / SKIPPED: 0
  Reference: TOTAL: 64 / PASSED: 8 / FAILED: 56 / SKIPPED: 0

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.07%. Comparing base (1710d9c) to head (dfc3c5f).

Files with missing lines	Patch %	Lines
src/sed/processor.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
- Coverage   82.11%   82.07%   -0.04%     
==========================================
  Files          13       13              
  Lines        5327     5329       +2     
  Branches      294      295       +1     
==========================================
  Hits         4374     4374              
- Misses        951      953       +2     
  Partials        2        2              

Flag	Coverage Δ
macos_latest	82.51% <0.00%> (-0.04%)	⬇️
ubuntu_latest	82.63% <0.00%> (-0.04%)	⬇️
windows_latest	0.00% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[sylvestre]

please add a comment explaining what it is doing