Don't test with one argument

[ci skip]

share/tools/web_config/sample_prompts/robbyrussell.fish

@@ -80,7 +80,8 @@ function fish_prompt
         set -l repo_branch $red(_repo_branch_name $repo_type)
         set repo_info "$blue $repo_type:($repo_branch$blue)"
 
-        if [ (_is_repo_dirty $repo_type) ]
+        set -l dirty (_is_repo_dirty $repo_type)
+        if test -n "$dirty"
             set -l dirty "$yellow ✗"
             set repo_info "$repo_info$dirty"
         end