Add custom marshallers for some ABAC types.

Javascript really does not like to get a null when it expects an empty array,
but that's what Go will send if your struct contains e.g. a []string and
you don't explicitly initialize the slice. These custom marshallers make sure
we send an empty array, making the frontend's life easier.

Go desperately needs some way to tell the JSON encoder that we want this
behavior, see:

golang/go#37711
golang/go#27589
golang/go#27813

client/types/abac.go

@@ -9,6 +9,7 @@
 package types
 
 import (
+	"encoding/json"
 	"errors"
 	"strings"
 	"time"
@@ -133,6 +134,16 @@ type CapabilityState struct {
 	Overrides []string
 }
 
+func (cs CapabilityState) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&struct {
+		Default   DefaultAccessRule
+		Overrides emptyStrings
+	}{
+		cs.Default,
+		cs.Overrides,
+	})
+}
+
 // CapabilityDesc is an enhanced structure containing a capability value, its name, and a brief description
 type CapabilityDesc struct {
 	Cap      Capability
@@ -780,6 +791,16 @@ type TagAccess struct {
 	Overrides []string //override sets an explicit allow or deny depending on Default state
 }
 
+func (ta TagAccess) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&struct {
+		Default   DefaultAccessRule
+		Overrides emptyStrings
+	}{
+		ta.Default,
+		ta.Overrides,
+	})
+}
+
 // check returns two values:
 //
 //	explicit: Whether or not the tag is in the TagAccess object.