bug: fix unmarshal consumer identity with empty value (#1057)

<!-- markdownlint-disable MD041 -->
#### What this PR does / why we need it
Fix empty identity value regression.

The regression was introduced after this
[PR](#927) adding a
custom unmarshaler for consumer identities which allows to specify
values other than string (e.g. port: 5000), even though consumer
identity is a map[string]string.


#### Which issue(s) this PR fixes
<!--
Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`.
-->

api/credentials/internal/identity.go

@@ -93,22 +93,32 @@ func orMatcher(list []IdentityMatcher) IdentityMatcher {
 // ConsumerIdentity describes the identity of a credential consumer.
 type ConsumerIdentity map[string]string
 
+// UnmarshalJSON allows a yaml specification containing a data type other
+// string, e.g. a hostpath spec with a port. Previously, it would error if the
+// user specified `port: 5000` and instead, the user had to specify
+// `port: "5000"`.
 func (c *ConsumerIdentity) UnmarshalJSON(data []byte) error {
 	var m map[string]interface{}
 	err := runtime.DefaultJSONEncoding.Unmarshal(data, &m)
 	if err != nil {
 		return err
 	}
 
+	if len(m) == 0 {
+		return nil
+	}
 	*c = make(map[string]string, len(m))
 	for k, v := range m {
 		switch v.(type) {
+		case nil:
+			(*c)[k] = ""
 		case map[string]interface{}:
 			return fmt.Errorf("cannot unmarshal complex type into consumer identity")
 		case []interface{}:
 			return fmt.Errorf("cannot unmarshal complex type into consumer identity")
+		default:
+			(*c)[k] = fmt.Sprintf("%v", v)
 		}
-		(*c)[k] = fmt.Sprintf("%v", v)
 	}
 	return nil
 }

api/credentials/internal/identity_test.go

@@ -70,4 +70,19 @@ port:
 		cid := internal.ConsumerIdentity{}
 		Expect(yaml.Unmarshal([]byte(data), &cid)).NotTo(Succeed())
 	})
+	It("with nil", func() {
+		data := `
+scheme: http
+hostname: 127.0.0.1
+port:
+`
+		id := internal.ConsumerIdentity{
+			"scheme":   "http",
+			"hostname": "127.0.0.1",
+			"port":     "",
+		}
+		cid := internal.ConsumerIdentity{}
+		Expect(yaml.Unmarshal([]byte(data), &cid)).To(Succeed())
+		Expect(cid).To(Equal(id))
+	})
 })