fix(changelog): empty lines at the beginning of the CHANGELOG

[woile]

Closes #192

Types of changes

Please put an x in the box that applies

• Bugfix
• New feature
• Refactoring
• Breaking change (any change that would cause existing functionality to not work as expected)
• Documentation Update
• Other (please describe)

Description

Empty lines at the beginning of the changelog

Checklist:

• Add test cases to all the changes you introduce
• Run ./script/lint and ./script/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
  Update the documentation for the changes

Steps to Test This Pull Request

Steps to reproduce the behavior:

1. Create a commit
2. Run cz changelog --incremental
3. Create a commit
4. Run cz changelog --incremental
5. Create a commit
6. Run cz changelog --incremental

Expected behavior

No new extra lines are added

Related Issue

#192

[codecov]

Codecov Report

Merging #193 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #193   +/-   ##
=======================================
  Coverage   92.14%   92.14%           
=======================================
  Files          35       35           
  Lines         967      968    +1     
=======================================
+ Hits          891      892    +1     
  Misses         76       76           

Flag	Coverage Δ
#unittests	92.14% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/changelog.py	91.89% <100.00%> (+0.11%)	⬆️

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[Lee-W]

LGTM 🎉