LSP: Short-circuit documentColors request for no servers

This fixes a deadlock when starting Helix with very many files, like
`hx runtime/queries/*/*.scm`. The tree-sitter query files don't have
an active language server on my machine and yet we were spawning a tokio
task to collect documentColors responses. We can skip that entirely.
Further debugging is needed to figure out why this lead to a deadlock
previously.

helix-term/src/handlers/document_colors.rs

@@ -81,6 +81,10 @@ fn request_document_colors(editor: &mut Editor, doc_id: DocumentId) {
         })
         .collect();
 
+    if futures.is_empty() {
+        return;
+    }
+
     tokio::spawn(async move {
         let mut all_colors = Vec::new();
         loop {