reflect/protodesc: don't panic on nil Resolver

The change to make protodesc.NewFile take an interface rather than a
concrete type means that NewFile(f, nil) now causes a panic. (A nil
*protoregistry.Files is valid.)

Fix this panic by using a default, empty registry when NewFile's second
parameter is nil.

Change-Id: I70a1f0759e7ea5b57fba5b6123ee85188f4d560c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/182979
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

reflect/protodesc/protodesc.go

@@ -13,6 +13,7 @@ import (
 	"google.golang.org/protobuf/internal/errors"
 	"google.golang.org/protobuf/internal/prototype"
 	"google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/reflect/protoregistry"
 
 	"google.golang.org/protobuf/types/descriptorpb"
 )
@@ -53,6 +54,9 @@ type Resolver interface {
 // The caller must relinquish full ownership of the input fd and must not
 // access or mutate any fields.
 func NewFile(fd *descriptorpb.FileDescriptorProto, r Resolver) (protoreflect.FileDescriptor, error) {
+	if r == nil {
+		r = (*protoregistry.Files)(nil) // empty resolver
+	}
 	var f prototype.File
 	switch fd.GetSyntax() {
 	case "proto2", "":