Fix warnings for uninstalled language servers

CONTRIBUTING.md

@@ -332,7 +332,7 @@ python scripts/lint.py
 
 ### Specs
 
-While language servers can be configured by the user using a simple JSON or Python [configuration file](./Configuring.html#language-servers),
+While language servers can be configured by the user using a simple JSON or Python [configuration file](./docs/Configuring.ipynb),
 it is preferable to provide users with an option that does not require manual configuration. The language server specifications (specs)
 wrap the configuration (as would be defined by the user) into a Python class or function that can be either:
 
@@ -361,7 +361,7 @@ A spec is a Python callable (a function, or a class with `__call__` method) that
 
 The above example is only intended as an illustration and not as an up-to-date guide.
 For details on the dictionary contents, see the [schema][] definition and [built-in specs][].
-Basic concepts (meaning of the `argv` and `languages` arguments) are also explained in the [configuration files](./Configuring.html#language-servers) documentation.
+Basic concepts (meaning of the `argv` and `languages` arguments) are also explained in the [configuration files](./docs/Configuring.ipynb) documentation.
 
 When contributing a specification we recommend to make use of the helper classes and other [utilities][] that take care of the common use-cases:
 

python_packages/jupyter_lsp/jupyter_lsp/specs/utils.py

@@ -141,9 +141,11 @@ def __call__(self, mgr: LanguageServerManagerAPI) -> KeyedLanguageServerSpecs:
             troubleshooting.append(spec["troubleshoot"])
         spec["troubleshoot"] = "\n\n".join(troubleshooting)
 
+        is_installed = self.is_installed(mgr)
+
         return {
             self.key: {
-                "argv": [mgr.nodejs, node_module, *self.args],
+                "argv": ([mgr.nodejs, node_module, *self.args] if is_installed else []),
                 "languages": self.languages,
                 "version": SPEC_VERSION,
                 **spec,