Only run npm pack if there are public packages in workspace (#522)

* Only run npm pack if there are public packages in workspace

* Add warning if workspaces does not contain public package

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

jupyter_releaser/npm.py

@@ -47,8 +47,12 @@ def build_dist(package, dist_dir):
             if data.get("private", False):
                 continue
             paths.append(str(osp.abspath(path)).replace(os.sep, "/"))
-
-        util.run(f"npm pack {' '.join(paths)}", cwd=dest, quiet=True)
+        if paths:
+            util.run(f"npm pack {' '.join(paths)}", cwd=dest, quiet=True)
+        else:
+            util.log(
+                "The NPM package defines 'workspaces' that does not contain any public package; this may be a mistake."
+            )
 
 
 def extract_dist(dist_dir, target):