Fixed issue with enter key selecting wrong option in dropdown

If you search for a label and press enter the dropdown will select the last element rather than the first. This is because the currentIndex is -1 rather than 0

Closes #17630

app/assets/javascripts/gl_dropdown.js.coffee

@@ -460,7 +460,7 @@ class GitLabDropdown
         return false
 
       if currentKeyCode is 13
-        @selectRowAtIndex currentIndex
+        @selectRowAtIndex if currentIndex < 0 then 0 else currentIndex
 
   removeArrayKeyEvent: ->
     $('body').off 'keydown'