ExpandedStruct panics with primitive types

[slinkydeveloper]

Reproducer:

import (
	"fmt"
	"github.com/invopop/jsonschema"
)

func main() {
	reflector := jsonschema.Reflector{ExpandedStruct: true}
	schema := reflector.Reflect("some string")

	fmt.Printf("%v", schema)
}

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4d8eb6]

goroutine 1 [running]:
github.com/invopop/jsonschema.(*Reflector).ReflectFromType(0xc0000b5eb8, {0x541dd8, 0x4ed2c0})
	/home/slinkydeveloper/go/pkg/mod/github.com/invopop/jsonschema@v0.13.0/reflect.go:191 +0x196
github.com/invopop/jsonschema.(*Reflector).Reflect(0x0?, {0x4ed2c0?, 0x53f418?})
	/home/slinkydeveloper/go/pkg/mod/github.com/invopop/jsonschema@v0.13.0/reflect.go:175 +0x45