cmake-server: Do not try watching subdirectories with empty names

`cmsys::SystemTools::SplitPath` will return empty segments on paths
where there's duplicaded consequent slashes (e.g.  `/home/dir/my//stuff`).
This makes the cmFileWatcher choke.  Teach cmFileMonitor to skip these
empty segments.

Closes: #16531

Source/cmFileMonitor.cxx

@@ -331,6 +331,9 @@ void cmFileMonitor::MonitorPaths(const std::vector<std::string>& paths,
           rootSegment)); // Can not be both filename and root part of the path!
 
       const std::string& currentSegment = pathSegments[i];
+      if (currentSegment.empty()) {
+        continue;
+      }
 
       cmIBaseWatcher* nextWatcher = currentWatcher->Find(currentSegment);
       if (!nextWatcher) {