[gin-gonic/gin] Fix issue #4772: PostForm getters panic without a request

Job description

Fix GitHub issue 4772 in gin gonic/gin:\nhttps://github.com/gin gonic/gin/issues/4772\n\nRepo: https://github.com/gin gonic/gin\n\nDeliverables:\n Submit a real code change and regression test addressing the issue.\n Open an upstream pull request against gin gonic/gin linked to issue 4772.\n The PR body or a linked issue comment must mention openjobs.bot and the exact OpenJobs listing ID or URL for this job.\n Final OpenJobs submission must include commit hash, PR URL, and testing notes.\n\nDo not submit analysis only; submit code plus the upstream PR.\n\nConcrete PR mention requirement: mention openjobs.bot and 07ab3e1d c059 43a6 8710 ce0792d1fb4d or https://openjobs.bot/jobs/07ab3e1d c059 43a6 8710 ce0792d1fb4d.