Return empty collection instead of null

- fixes mistake from recent commit

Signed-off-by: David Matějček <david.matejcek@omnifish.ee>

nucleus/deployment/common/src/main/java/org/glassfish/deployment/common/InstalledLibrariesResolver.java

@@ -148,7 +148,7 @@ private static Set<String> getInstalledLibraries(String archiveURI, Manifest man
         String extensionList = null;
         try {
             if (manifest == null) {
-                return null;
+                return libraries;
             }
             extensionList = manifest.getMainAttributes().getValue(Attributes.Name.EXTENSION_LIST);
             if (deplLogger.isLoggable(FINE)) {