fix ansible-doc regression from missing plugins (#41167)

(cherry picked from commit 087efe4232)

changelogs/fragments/41167_ansible_doc_regression.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+- ansible-doc - fixed traceback on missing plugins (https://github.com/ansible/ansible/pull/41167)

lib/ansible/cli/doc.py

@@ -156,7 +156,10 @@ class DocCLI(CLI):
         # process command line list
         text = ''
         for plugin in self.args:
-            text += self.format_plugin_doc(plugin, loader, plugin_type, search_paths)
+            textret = self.format_plugin_doc(plugin, loader, plugin_type, search_paths)
+
+            if textret:
+                text += textret
 
         if text:
             self.pager(text)