Github API error when no commits exist between branches

[commscheck]

Steps to reproduce

Create .pr-train.yml:

trains:
  appsflyer:
    - ben-appsflyer-archx
    - ben-appsflyer-activation-events

Run the following:

git checkout -b branch-a
# make changes
git commit -a -m "commit message"
git checkout -b branch-b
git pr-train --create-prs

Expected

pr-train checks if all branches in a chain have different HEADs before pushing to Github

Actual

{
  "message": "Validation Failed",
  "statusCode": 422,
  "headers": {
    "server": "GitHub.com",
    "date": "Fri, 22 Oct 2021 04:49:49 GMT",
    "content-type": "application/json; charset=utf-8",
    "content-length": "261",
    "x-oauth-scopes": "repo",
    "x-accepted-oauth-scopes": "",
    "x-github-media-type": "github.v3; format=json",
    "x-ratelimit-limit": "5000",
    "x-ratelimit-remaining": "4962",
    "x-ratelimit-reset": "1634878448",
    "x-ratelimit-used": "38",
    "x-ratelimit-resource": "core",
    "access-control-expose-headers": "ETag, Link, Location, Retry-After, X-GitHub-OTP, X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Used, X-RateLimit-Resource, X-RateLimit-Reset, X-OAuth-Scopes, X-Accepted-OAuth-Scopes, X-Poll-Interval, X-GitHub-Media-Type, Deprecation, Sunset",
    "access-control-allow-origin": "*",
    "strict-transport-security": "max-age=31536000; includeSubdomains; preload",
    "x-frame-options": "deny",
    "x-content-type-options": "nosniff",
    "x-xss-protection": "0",
    "referrer-policy": "origin-when-cross-origin, strict-origin-when-cross-origin",
    "content-security-policy": "default-src 'none'",
    "vary": "Accept-Encoding, Accept, X-Requested-With",
    "x-github-request-id": "REDACTED",
    "connection": "close"
  },
  "body": {
    "message": "Validation Failed",
    "errors": [
      {
        "resource": "PullRequest",
        "code": "custom",
        "message": "No commits between branch-a and branch-b"
      }
    ],
    "documentation_url": "https://docs.github.com/rest/reference/pulls#create-a-pull-request"
  }
}