fix(lsp): empty filterText/sortText/insertText not treated as unset #41421

Problem:
Empty string is falsy, so the spec's label fallback applies, but there only checks
for nil. An empty filterText drops the item, an empty sortText sorts it
first, and an empty insertText leaves the word empty.

Solution:
Treat empty string as unset.

runtime/lua/vim/lsp/completion.lua

@@ -160,6 +160,12 @@ local function has_completeopt(flag)
   return vim.list_contains(vim.opt.completeopt:get(), flag)
 end
 
+--- @param s string?
+--- @return string?
+local function nonempty(s)
+  return s ~= '' and s or nil
+end
+
 --- @param input string Unparsed snippet
 --- @return string # Parsed snippet if successful, else returns its input
 local function parse_snippet(input)
@@ -211,12 +217,10 @@ local function get_completion_word(item, prefix, match)
       --    label: insert
       --
       -- Typing `i` would remove the candidate because newText starts with `t`.
-      local text = parse_snippet(item.insertText or item.textEdit.newText)
+      local text = parse_snippet(nonempty(item.insertText) or item.textEdit.newText)
+      local filter_text = nonempty(item.filterText)
       local word = #text < #item.label and vim.fn.matchstr(text, '\\k*')
-        or (
-          item.filterText and vim.fn.match(item.label, '^\\k') == -1 and item.filterText
-          or item.label
-        )
+        or (filter_text and vim.fn.match(item.label, '^\\k') == -1 and filter_text or item.label)
       return fallback_filtertext(item, word, prefix, match)
     else
       return item.label
@@ -266,7 +270,10 @@ local function apply_defaults(item, defaults, apply_kind)
   if defaults.editRange then
     local textEdit = item.textEdit or {}
     item.textEdit = textEdit
-    textEdit.newText = textEdit.newText or item.textEditText or item.insertText or item.label
+    textEdit.newText = textEdit.newText
+      or item.textEditText
+      or nonempty(item.insertText)
+      or item.label
     if defaults.editRange.start then
       textEdit.range = textEdit.range or defaults.editRange
     elseif defaults.editRange.insert then
@@ -481,8 +488,9 @@ function M._lsp_to_complete_items(
   else
     ---@param item lsp.CompletionItem
     matches = function(item)
-      if item.filterText then
-        return match_item_by_value(item.filterText, prefix)
+      local filter_text = nonempty(item.filterText)
+      if filter_text then
+        return match_item_by_value(filter_text, prefix)
       end
 
       if item.textEdit and not item.textEdit.newText then
@@ -593,7 +601,7 @@ function M._lsp_to_complete_items(
       local itema = a.user_data.nvim.lsp.completion_item
       ---@type lsp.CompletionItem
       local itemb = b.user_data.nvim.lsp.completion_item
-      return (itema.sortText or itema.label) < (itemb.sortText or itemb.label)
+      return (nonempty(itema.sortText) or itema.label) < (nonempty(itemb.sortText) or itemb.label)
     end
 
     local use_fuzzy_sort = has_completeopt('fuzzy')

test/functional/plugin/lsp/completion_spec.lua

@@ -135,6 +135,14 @@ describe('vim.lsp.completion: item conversion', function()
           range = range0,
         },
       },
+      -- "" is unset
+      {
+        label = 'zoocar_long',
+        sortText = '',
+        insertText = '',
+        insertTextFormat = 2,
+        textEdit = { newText = 'foobar($1)', range = range0 },
+      },
     }
     local expected = {
       { abbr = 'foobar', word = 'foobar' },
@@ -145,6 +153,7 @@ describe('vim.lsp.completion: item conversion', function()
       { abbr = 'foocar', word = 'foobar' },
       { abbr = 'foocar', word = 'foodar(${1:var1})' }, -- marked as PlainText, text is used as is
       { abbr = '•INT16_C(c)', word = 'INT16_C' },
+      { abbr = 'zoocar_long', word = 'foobar' },
     }
     local result = complete('|', completion_list)
     eq(expected, extract_word_abbr(result.items))
@@ -299,9 +308,9 @@ describe('vim.lsp.completion: item conversion', function()
       })
     end)
 
-    it('fuzzy matches on label when filterText is missing', function()
+    it('fuzzy matches on label when filterText is missing or empty', function()
       assert_completion_matches('fo', {
-        { label = 'foo' },
+        { label = 'foo', filterText = '' },
         { label = 'faz other' },
         { label = 'bar' },
       }, {
@@ -755,6 +764,7 @@ describe('vim.lsp.completion: item conversion', function()
           {
             label = 'hello',
             data = 'item-property-has-priority',
+            insertText = '',
           },
         },
       }