PostForm getters panic when Context has no request

Description

Context.GetPostForm, Context.PostForm, and Context.DefaultPostForm panic when c.Request is nil. This differs from the equivalent query getters, which return empty values safely.

Reproduction

c, _ := gin.CreateTestContext(httptest.NewRecorder())
c.PostForm("key") // panic: nil pointer dereference

Expected behavior

The form getters should behave like the query getters for an empty context:

• GetPostForm("key") returns ("", false)
• PostForm("key") returns ""
• DefaultPostForm("key", "fallback") returns "fallback"

Actual behavior

initFormCache calls c.Request.ParseMultipartForm(...) without checking whether c.Request is nil, causing a nil-pointer panic.

Proposed fix

Initialize an empty form cache and return when there is no request. This preserves normal request behavior and matches initQueryCache.

Validation

A regression test covering all three getters passes, along with go test ./... and go vet ./....

Environment

• Gin: master at 34dac20
• Go: 1.26.4
• OS: macOS