Issue #3356052: KeyProviderInterface::getKeyValue() doesn't always return a string

src/Plugin/KeyProvider/EnvKeyProvider.php

@@ -92,7 +92,7 @@ class EnvKeyProvider extends KeyProviderBase implements KeyPluginFormInterface {
   /**
    * {@inheritdoc}
    */
-  public function getKeyValue(KeyInterface $key) {
+  public function getKeyValue(KeyInterface $key): ?string {
     $env_variable = $this->configuration['env_variable'];
     $key_value = getenv($env_variable);
 
@@ -108,7 +108,7 @@ class EnvKeyProvider extends KeyProviderBase implements KeyPluginFormInterface {
       $key_value = base64_decode($key_value);
     }
 
-    return $key_value;
+    return (string) $key_value;
   }
 
 }