Config: get rid of default registry values

All default values are empty strings, and the linter was also
complaining about readValue always receiving the same value for its
'defaultValue' argument.

Hence, I simplified the code so that there are no default values, or
rather the default values are always "". We can bring them back if we
ever need one of the defaults to be non-empty.

windows-agent/internal/config/config.go

@@ -19,16 +19,9 @@ import (
 const (
 	registryPath = `Software\Canonical\UbuntuPro`
 
-	defaultToken = ""
-
-	fieldLandscapeClientConfig   = "LandscapeClientConfig"
-	defaultLandscapeClientConfig = ""
-
-	fieldLandscapeAgentURL   = "LandscapeAgentURL"
-	defaultLandscapeAgentURL = ""
-
-	fieldLandscapeAgentUID   = "LandscapeAgentUID"
-	defaultLandscapeAgentUID = ""
+	fieldLandscapeClientConfig = "LandscapeClientConfig"
+	fieldLandscapeAgentURL     = "LandscapeAgentURL"
+	fieldLandscapeAgentUID     = "LandscapeAgentUID"
 )
 
 // fieldsProToken contains the fields in the registry where each source will store its token.
@@ -304,9 +297,6 @@ func (c *Config) loadRegistry(ctx context.Context) (proTokens map[SubscriptionSo
 	k, err := c.registry.HKCUOpenKey(registryPath, registry.READ)
 	if errors.Is(err, registry.ErrKeyNotExist) {
 		log.Debug(ctx, "Registry key does not exist, using default values")
-		data.landscapeAgentURL = defaultLandscapeAgentURL
-		data.landscapeClientConfig = defaultLandscapeClientConfig
-		data.landscapeAgentUID = defaultLandscapeAgentUID
 		return proTokens, data, nil
 	}
 	if err != nil {
@@ -315,7 +305,7 @@ func (c *Config) loadRegistry(ctx context.Context) (proTokens map[SubscriptionSo
 	defer c.registry.CloseKey(k)
 
 	for source, field := range fieldsProToken {
-		proToken, e := c.readValue(ctx, k, field, defaultToken)
+		proToken, e := c.readValue(ctx, k, field)
 		if e != nil {
 			err = errors.Join(err, fmt.Errorf("could not read %q: %v", field, e))
 			continue
@@ -332,29 +322,29 @@ func (c *Config) loadRegistry(ctx context.Context) (proTokens map[SubscriptionSo
 		return nil, data, err
 	}
 
-	data.landscapeAgentURL, err = c.readValue(ctx, k, fieldLandscapeAgentURL, defaultLandscapeAgentURL)
+	data.landscapeAgentURL, err = c.readValue(ctx, k, fieldLandscapeAgentURL)
 	if err != nil {
 		return proTokens, data, err
 	}
 
-	data.landscapeClientConfig, err = c.readValue(ctx, k, fieldLandscapeClientConfig, defaultLandscapeClientConfig)
+	data.landscapeClientConfig, err = c.readValue(ctx, k, fieldLandscapeClientConfig)
 	if err != nil {
 		return proTokens, data, err
 	}
 
-	data.landscapeAgentUID, err = c.readValue(ctx, k, fieldLandscapeAgentUID, defaultLandscapeAgentUID)
+	data.landscapeAgentUID, err = c.readValue(ctx, k, fieldLandscapeAgentUID)
 	if err != nil {
 		return proTokens, data, err
 	}
 
 	return proTokens, data, nil
 }
 
-func (c *Config) readValue(ctx context.Context, key uintptr, field string, defaultValue string) (string, error) {
+func (c *Config) readValue(ctx context.Context, key uintptr, field string) (string, error) {
 	value, err := c.registry.ReadValue(key, field)
 	if errors.Is(err, registry.ErrFieldNotExist) {
-		log.Debugf(ctx, "Registry value %q does not exist, defaulting to %q", field, defaultValue)
-		return defaultValue, nil
+		log.Debugf(ctx, "Registry value %q does not exist, defaulting to empty", field)
+		return "", nil
 	}
 	if err != nil {
 		return "", err