Revert "api/types/registry: EncodeAuthConfig: use empty string for zero value"

This reverts commit 3a447bc079.

Some daemon versions don't handle empty values well, which resulted in
an io.EOF error when sending an empty X-Registry-Auth during decoding
or unmarshaling.

We should investigate what code-paths are hit to trigger this, but
in the meantime, let's revert this patch.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

api/types/registry/authconfig.go

@@ -52,9 +52,6 @@ type AuthConfig struct {
 //
 // [RFC4648, section 5]: https://tools.ietf.org/html/rfc4648#section-5
 func EncodeAuthConfig(authConfig AuthConfig) (string, error) {
-	if authConfig == (AuthConfig{}) {
-		return "", nil
-	}
 	buf, err := json.Marshal(authConfig)
 	if err != nil {
 		return "", errInvalidParameter{err}

api/types/registry/authconfig_test.go

@@ -111,8 +111,8 @@ func TestEncodeAuthConfig(t *testing.T) {
 		{
 			doc:       "empty",
 			input:     AuthConfig{},
-			outBase64: ``,
-			outPlain:  ``,
+			outBase64: `e30=`,
+			outPlain:  `{}`,
 		},
 		{
 			doc: "test authConfig",