### Issue
Fully publishing a draft release fails when the release does not have a
target set.
A draft release can be created without a target because the Git tag is
not created while the release remains a draft. However, when the draft
is published, Gitea attempts to resolve the release target to create the
tag. If the target is empty, the publish operation fails.
### Solution
Use the repository's default branch as the target when a draft release
is published without an explicitly configured target.
Also add a regression test to verify that a draft release without a
target can be successfully published.
Fixes#35569
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Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
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