fix(openapi): don't panic on requestBody content mime without schema

[rinor]

Panics in cases like:

      requestBody:
        required: true
        content:
          application/json: {}

• gptscript --list-tools https://github.com/raw/cloudflare/api-schemas/main/openapi.yaml

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x9d209e]

goroutine 1 [running]:
github.com/gptscript-ai/gptscript/pkg/loader.getOpenAPITools(0xc0000e1ef0, {0xc0001900a0, 0x4a}, {0xc0001900a0, 0x4a}, {0x0, 0x0})
	github.com/gptscript-ai/gptscript/pkg/loader/openapi.go:212 +0x2b7e
...

[g-linville]

Thanks for finding this, @rinor !