[Bug] [API] Get PullRequest no SHA of HEAD

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gitea: v1.12.3

if pull is from an fork and merged/closed and branch deleted:

api responce has no SHA

ToDo: need to check if there are more edgecases who cause the same

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sidenode: this bug caused #12657 to create pulls who only show 500 page <- so the localgiteauploader should have some savety check ...
workaround is to query the commit by ref and use that sha

[stale]

This issue has been automatically marked as stale because it has not had recent activity. I am here to help clear issues left open even if solved or waiting for more insight. This issue will be closed if no further activity occurs during the next 2 weeks. If the issue is still valid just add a comment to keep it alive. Thank you for your contributions.

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for those looking for a workaround code: https://gitea.com/gitea/tea/pulls/340 or #14922