Add a null check for a method return result that may return null (#9354)

* Add a null check for a method return result that may return null.

In file PluginManager.java the parsePluginManifest method may return null. In other parts of the code (https://github.com/jenkinsci/jenkins/blob/7aad67832442b8cd1dfaf363a3630d2687ddf3e3/core/src/main/java/hudson/PluginManager.java#L868), the return value was checked for null. But the null check is missing in this part of the code. 

The fix just adds the missing null check in the code.

**Sponsorship and Support:**

This work is done by the security researchers from OpenRefactory and is supported by the Open Source Security Foundation (OpenSSF): Project Alpha-Omega. Alpha-Omega is a project partnering with open source software project maintainers to systematically find new, as-yet-undiscovered vulnerabilities in open source code - and get them fixed - to improve global software supply chain security.

The bug is found by running the iCR tool by OpenRefactory, Inc. and then manually triaging the results.

* Comment: Applied mvn spotless to clean up formating issues.

---------

Co-authored-by: Munawar Hafiz <munawar@munawars-mbp.lan>

core/src/main/java/hudson/PluginManager.java

@@ -716,6 +716,10 @@ protected static void addDependencies(URL hpiResUrl, String fromPath, Set<URL> d
         }
 
         Manifest manifest = parsePluginManifest(hpiResUrl);
+        if (manifest == null) {
+            return;
+        }
+
         String dependencySpec = manifest.getMainAttributes().getValue("Plugin-Dependencies");
         if (dependencySpec != null) {
             String[] dependencyTokens = dependencySpec.split(",");