Fix tests and update to release dependency

* Align -mock module with kubernetes-client. Since it pulls junit5, the
  vintage module is required to keep running junit 4 tests
* Fluent api doesn't initialize empty structure anymore

pom.xml

@@ -47,6 +47,7 @@
     <changelist>-SNAPSHOT</changelist>
     <jenkins.version>2.222.4</jenkins.version>
     <java.level>8</java.level>
+    <junit5.version>5.7.2</junit5.version>
   </properties>
 
   <scm>
@@ -242,7 +243,7 @@
     <dependency>
       <groupId>io.fabric8</groupId>
       <artifactId>kubernetes-server-mock</artifactId>
-      <version>4.6.0</version>
+      <version>5.4.1</version>
       <scope>test</scope>
       <exclusions>
         <!-- fails maven enforcer because kubernetes-client has a different version
@@ -294,4 +295,19 @@
       <url>https://repo.jenkins-ci.org/public/</url>
     </pluginRepository>
   </pluginRepositories>
+
+  <build>
+    <plugins>
+      <plugin>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <dependencies>
+          <dependency>
+            <groupId>org.junit.vintage</groupId>
+            <artifactId>junit-vintage-engine</artifactId>
+            <version>${junit5.version}</version>
+          </dependency>
+        </dependencies>
+      </plugin>
+    </plugins>
+  </build>
 </project>

src/main/java/com/cloudbees/jenkins/plugins/kubernetes_credentials_provider/SecretUtils.java

@@ -196,10 +196,8 @@ public static <T> T requireNonNull(@Nullable T obj, String exceptionMessage, @Nu
     @SuppressFBWarnings(value= {"ES_COMPARING_PARAMETER_STRING_WITH_EQ"}, justification="the string will be the same string if not mapped")
     public static String getNonNullSecretData(Secret s, String key, String exceptionMessage) throws CredentialsConvertionException {
         String mappedKey = getKeyName(s, key);
-        if (mappedKey == key) { // use String == as getKeyName(key) will return key if no custom mapping is defined)
-            return requireNonNull(s.getData().get(key), exceptionMessage, null);
-        }
-        return requireNonNull(s.getData().get(mappedKey), exceptionMessage, mappedKey);
+        Map<String, String> data = requireNonNull(s.getData(), exceptionMessage, mappedKey == key ? null : mappedKey);
+        return requireNonNull(data.get(mappedKey), exceptionMessage, mappedKey);
     }
 
     /**