Remove omitempty from Content field of RepositoryContentFileOptions

[Ashvin-Ranjan]

fixes #2427

This removes omitempty from RepositoryContentFileOptions.Content so that an empty content field can be passed in to generate empty files. Before, the content field was omitted, which caused the GitHub API to not accept the request as the field is required, even if it is empty.

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[codecov]

Codecov Report

Merging #2430 (5498f4f) into master (3a432d6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2430   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files         120      120           
  Lines       10558    10558           
=======================================
  Hits        10354    10354           
  Misses        140      140           
  Partials       64       64           

Impacted Files	Coverage Δ
github/repos_contents.go	89.33% <ø> (ø)

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[gmlewis]

Thank you, @Ashvin-Ranjan !
LGTM.
Merging.