internal/golang: CodeAction: don't return empty source.doc titles

The source.doc code action was observed to produce empty command
titles on occasion; it should not offer a command in that case.

(A test doesn't make much sense because it would assert that a
particular input doesn't produce a command, whereas the invariant
that matters is that empty command titles are never shown.)

Change-Id: Ibe5de418954d299ea0d7809c35e7496ff353be6f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/639836
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/golang/codeaction.go

@@ -442,8 +442,10 @@ func goplsDocFeatures(ctx context.Context, req *codeActionsRequest) error {
 // See [server.commandHandler.Doc] for command implementation.
 func goDoc(ctx context.Context, req *codeActionsRequest) error {
 	_, _, title := DocFragment(req.pkg, req.pgf, req.start, req.end)
-	cmd := command.NewDocCommand(title, command.DocArgs{Location: req.loc, ShowDocument: true})
-	req.addCommandAction(cmd, false)
+	if title != "" {
+		cmd := command.NewDocCommand(title, command.DocArgs{Location: req.loc, ShowDocument: true})
+		req.addCommandAction(cmd, false)
+	}
 	return nil
 }