fix: init string before show cached help

Author: DanielMicrosoft

src/azure-cli-core/azure/cli/core/commands/__init__.py

@@ -748,9 +748,15 @@ def _try_show_cached_help(self, args):
         if not help_index and command_index.needs_latest_extension_help_overlay_refresh():
             logger.debug("Top-level cached help is unavailable on latest profile. "
                          "Refreshing extension help overlay without full core module load.")
-            # Unknown top-level command forces extension-only load path on latest profile.
-            self.commands_loader.load_command_table(['__refresh_extension_help_overlay__'])
-            help_index = command_index.get_help_index()
+            try:
+                if self.cli_ctx.invocation.data.get('command_string') is None:
+                    self.cli_ctx.invocation.data['command_string'] = ''
+                # Unknown top-level command forces extension-only load path on latest profile.
+                self.commands_loader.load_command_table(['__refresh_extension_help_overlay__'])
+                help_index = command_index.get_help_index()
+            except Exception as ex:  # pylint: disable=broad-except
+                # Keep cached-help refresh best-effort; normal invocation flow can still continue.
+                logger.debug("Failed to refresh latest extension help overlay: %s", ex)
 
         if help_index:
             # Display cached help using the help system