gptel-integrations: Guard activate-tools against nil tools list

When `gptel-mcp-connect` is called for specific servers that fail to
start or return no tools, the `add-all-tools` callback passes nil to
`gptel-mcp--activate-tools`.  Its `(unless tools ...)` fallback then
fetches tools from ALL connected MCP servers and tries to look them up
via `gptel-get-tool`, crashing on tools from servers that were never
registered with `gptel-make-tool`.

Guard the call with `when` so nil means "nothing to activate" rather
than "activate everything".

Author: benthamite

gptel-integrations.el

@@ -163,7 +163,7 @@ not a function and non-nil, start SERVERS synchronously."
                                   (or server-names (mapcar #'car inactive-servers))))
                           (now-active (cl-remove-if-not server-active-p mcp-hub-servers)))
                       (mapc (lambda (tool) (apply #'gptel-make-tool tool)) tools)
-                      (gptel-mcp--activate-tools tools)
+                      (when tools (gptel-mcp--activate-tools tools))
                       (if-let* ((failed (cl-set-difference inactive-servers now-active
                                                            :test #'equal)))
                           (progn