createFile bug

[zhengrenjie]

https://github.com/gmessner/gitlab4j-api/blob/69f26d017d4816fbe5e93d23b91d83280b9745df/src/main/java/org/gitlab4j/api/AbstractApi.java#L402
When i want to create a new file with empty content , a exception thrown . But gitlab web and gitlab api (has tested by postman)both allow the empty content.
I think this might be a bug , but this method has too many associations( i can't fix it) .

[gmessner]

@zhengrenjie
Because https://docs.gitlab.com/ee/api/repository_files.html#create-new-file-in-repository states that content is required, it was assumed that content could not be empty. I have verified that the GitLab API accepts empty content and fixed both createFile() and updateFile() to allow for empty or whitespace only content. Content cannot be null, but it can now be empty or whitespace only.

Release 4.8.24 is now available with the fix. When you have a chance, please verify and close this issue.

[zhengrenjie]

@gmessner That would be very grateful !