Fix bugs relating to optional fields with custom (un)marshalers

docs/CHANGELOG.md

@@ -34,6 +34,7 @@ When releasing a new version:
 
 - The `omitempty` option now works correctly for struct- and map-typed variables, matching `encoding/json`, which is to say it never omits structs, and omits empty maps. (#43)
 - Generated type-names now abbreviate across multiple components; for example if the path to a type is `(MyOperation, Outer, Outer, Inner, OuterInner)`, it will again be called `MyOperationOuterInner`.  (This regressed in a pre-v0.1.0 refactor.) (#109)
+- Previously, interface fields with `# @genqlient(pointer: true)` would be unmarshaled to `(*MyInterface)(*<nil>)`, i.e. a pointer to the untyped-nil of the interface type.  Now they are unmarshaled as `(*MyInterface)(<nil>)`, i.e. a nil pointer of the pointer-to-interface type, as you would expect.
 
 ## v0.1.0
 

generate/marshal.go.tmpl

@@ -30,9 +30,15 @@ func (v *{{.GoName}}) MarshalJSON() ([]byte, error) {
         for i, src := range src {
             dst := &(*dst)[i]
         {{end -}}
+        {{/* src now has type <GoType>; dst is json.RawMessage */ -}}
+        {{if $field.GoType.IsPointer -}}
+        {{/* If you passed a pointer, and it's nil, don't call the
+             marshaler.  This matches json.Marshal's behavior. */ -}}
+        if src != nil {
+        {{end -}}
         var err error
         *dst, err = {{$field.Marshaler $.Generator}}(
-            {{/* src is a pointer to the struct-field (or field-element, etc.).
+            {{/* src is the struct-field (or field-element, etc.).
                  We want to pass a pointer to the type you specified, so if
                  there's a pointer on the field that's exactly what we want,
                  and if not we need to take the address. */ -}}
@@ -41,6 +47,9 @@ func (v *{{.GoName}}) MarshalJSON() ([]byte, error) {
             return nil, fmt.Errorf(
                 "Unable to marshal {{$.GoName}}.{{$field.GoName}}: %w", err)
         }
+        {{if $field.GoType.IsPointer -}}
+        }{{/* end if src != nil */}}
+        {{end -}}
         {{range $i := intRange $field.GoType.SliceDepth -}}
         }
         {{end -}}