Create Tree no longer accepts null value in sha field

[asthinasthi]

Describe the bug
https://developer.github.com/v3/git/trees/#create-a-tree allows content field or sha field. You could not use both but you could send sha with null value if you populated the content field

https://developer.github.com/enterprise/2.16/v3/git/trees/

The SHA1 checksum ID of the object in the tree. Also called tree.sha.

Note: Use either tree.sha or content to specify the contents of the entry. Using both tree.sha and content will return an error.

With 2.17+ API's
https://developer.github.com/enterprise/2.17/v3/git/trees/

The SHA1 checksum ID of the object in the tree. Also called tree.sha. If the value is null then the file will be deleted.

Note: Use either tree.sha or content to specify the contents of the entry. Using both tree.sha and content will return an error.

In the library a sha field is always sent with null value even if we choose to just send the content field because of serialization of the TreeEntry fields.
https://github.com/github-api/github-api/blob/d1507f26668950508e0bf242c34cdb599003991a/src/main/java/org/kohsuke/github/GHTreeBuilder.java#L24

To Reproduce

1. Create a sample repo
2. Use https://github.com/github-api/github-api/blob/d1507f26668950508e0bf242c34cdb599003991a/src/main/java/org/kohsuke/github/GHTreeBuilder.java#L111 to add an entry (no sha required)
3. Invoke https://github.com/github-api/github-api/blob/d1507f26668950508e0bf242c34cdb599003991a/src/main/java/org/kohsuke/github/GHTreeBuilder.java#L164

The curl equivalent is:

curl -X POST \
  https://github.com/api/v3/repos/org/repo/git/trees \
  -H 'Authorization: token <>' \
  -H 'Content-Type: application/json' \
  -d '{
    "base_tree": "base_sha",
    "tree": [
        {
            "path": "somePath",
            "mode": "100644",
            "type": "blob",
            "sha": null,
            "content": "someText"
        }
    ]
}'

Expected behavior
Tree must be created but instead we see:

{
    "message": "Must supply tree.sha or tree.content",
    "documentation_url": "https://developer.github.com/enterprise/2.18/v3/git/trees/#create-a-tree"
}

Desktop (please complete the following information):
Latest version

Additional context

Adding a @JsonInclude(Include.NON_NULL) in https://github.com/github-api/github-api/blob/d1507f26668950508e0bf242c34cdb599003991a/src/main/java/org/kohsuke/github/GHTreeBuilder.java#L20 will fix the issue according to me

    @JsonInclude(Include.NON_NULL)
    private static final class TreeEntry {
        private final String path;
        private final String mode;
        private final String type;
        private String sha;
        private String content;

        private TreeEntry(String path, String mode, String type) {
            this.path = path;
            this.mode = mode;
            this.type = type;
        }
    }

[asthinasthi]

Suggested changes #642

[asthinasthi]

Method is deprecated. So closing this.

[bitwiseman]

Fixed in #642.