pluginUtility merge throws NullPointerException when input plugin-cfg.xml was generated with paused httpEndpoint listeners

[dhenriques]

Describe the bug
pluginUtility merge throws a NullPointerException when one of the input plugin-cfg.xml files
was generated while the server's httpEndpoint listeners were paused. When listeners are paused,
PluginGenerator writes a comment indicating the endpoint could not be found and skips the
<ServerCluster> element, but still emits <UriGroup> and <Route> unconditionally. During a
subsequent merge, PluginMergeToolImpl$PluginInfo.<init> performs a hashtable lookup for the
ServerCluster by name, receives null (because no <ServerCluster> exists in the XML), and
throws a NullPointerException.

java.lang.RuntimeException: java.lang.NullPointerException
	at com.ibm.ws.http.plugin.merge.internal.PluginMergeToolImpl.merge(PluginMergeToolImpl.java:776)
	at com.ibm.ws.webserver.plugin.utility.tasks.MergePluginFilesTask.merge(MergePluginFilesTask.java:204)
	at com.ibm.ws.webserver.plugin.utility.tasks.MergePluginFilesTask.mergePluginFilesFromList(MergePluginFilesTask.java:138)
	at com.ibm.ws.webserver.plugin.utility.tasks.MergePluginFilesTask.execute(MergePluginFilesTask.java:66)
	at com.ibm.ws.webserver.plugin.utility.WebServerPluginUtility.runProgram(WebServerPluginUtility.java:103)
	at com.ibm.ws.webserver.plugin.utility.WebServerPluginUtility.main(WebServerPluginUtility.java:146)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:572)
	at com.ibm.ws.kernel.boot.cmdline.UtilityMain.internal_main(UtilityMain.java:175)
	at com.ibm.ws.kernel.boot.cmdline.UtilityMain.main(UtilityMain.java:55)
	at com.ibm.ws.kernel.boot.cmdline.Main.main(Main.java:54)
Caused by: java.lang.NullPointerException
	at com.ibm.ws.http.plugin.merge.internal.PluginMergeToolImpl$PluginInfo.<init>(PluginMergeToolImpl.java:1093)
	at com.ibm.ws.http.plugin.merge.internal.PluginMergeToolImpl.loadData(PluginMergeToolImpl.java:570)
	at com.ibm.ws.http.plugin.merge.internal.PluginMergeToolImpl.merge(PluginMergeToolImpl.java:774)
	... 12 more

Steps to Reproduce

1. Configure a server with webServerPluginConfig-1.0 and an httpEndpoint.
2. Start the server, then pause the httpEndpoint listeners.
3. While listeners are paused, trigger plugin config generation (via pluginUtility generate or
   auto-generation). This produces a plugin-cfg.xml containing <UriGroup> and <Route> but
   no <ServerCluster> element, and the comment:

   <!-- The configured endpoint could not be found. httpEndpointRef=defaultHttpEndpoint-->

   (Note: Liberty generates this comment with a leading space inside the <!--.)
4. Run pluginUtility merge with the paused-endpoint file as one of the inputs:

   pluginUtility merge \
     --sourcePath=srv01-plugin-cfg.xml,srv02-plugin-cfg-paused-endpoint.xml \
     --targetPath=merged-out.xml
   

5. Observe NullPointerException as shown above.

The problematic input file has the following structure:

<?xml version="1.0" encoding="UTF-8"?>
<Config ASDisableNagle="false" AcceptAllContent="false" ...>
   <!-- The configured endpoint could not be found. httpEndpointRef=defaultHttpEndpoint-->
   <VirtualHostGroup Name="default_host">
      <VirtualHost Name="*:9080"/>
      <VirtualHost Name="*:9443"/>
   </VirtualHostGroup>
   <UriGroup Name="default_host_srv02_default_node_Cluster_URIs">
      <Uri AffinityCookie="JSESSIONID" AffinityURLIdentifier="jsessionid" Name="/myapp/*"/>
   </UriGroup>
   <Route ServerCluster="srv02_default_node_Cluster"
          UriGroup="default_host_srv02_default_node_Cluster_URIs"
          VirtualHostGroup="default_host"/>
</Config>

Expected behavior
pluginUtility merge should handle an input plugin-cfg.xml with no <ServerCluster> element
gracefully. Skipping that server's entries with a descriptive informational message, rather than
throwing an unhandled NullPointerException and aborting the entire merge.

Diagnostic information:

• OpenLiberty Version: All
• Affected feature(s): pluginUtility merge
• Java Version: N/A
• server.xml configuration: N/A

Additional context
Workaround: Resume listeners, regenerate plugin-cfg.xml, then re-run the merge.