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.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[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)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[gmlewis]

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