auto: do not create anonymous snippet

Do not create an anonymous snippet if there are no placeholders, because
there is no point in creating the snippet in that case and doing so
breaks Ultisnip navigation when the previous completion had
placeholders.

Fixes #3601

autoload/go/auto.vim

@@ -35,6 +35,15 @@ function! s:ExpandSnippet() abort
     " the snippet may have a '{\}' in it. For UltiSnips, that should be spelled
     " \{}. fmt.Printf is such a snippet that can be used to demonstrate.
     let l:snippet = substitute(v:completed_item.word, '{\\}', '\{}', 'g')
+
+    " If there are no placeholders in the returned text, then do not expand
+    " the snippet, because Ultisnip does not support nested snippets, and the
+    " user may already be in an expanded snippet. While this isn't foolproof,
+    " because there may be placeholders in the snippet, it at least solves the
+    " simple case.
+    if stridx(l:snippet, '$') == -1
+      return
+    endif
     call UltiSnips#Anon(l:snippet, v:completed_item.word, '', 'i')
 "  elseif l:engine is 'neosnippet'
 "    " TODO(bc): make the anonymous expansion for neosnippet work