Fix crash loop on bib entries with the cite key on its own line

lua/cmp_vimtex/parser.lua

@@ -208,6 +208,19 @@ parser.parse_string = function(self, line)
 end
 
 parser.parse_entry = function(self, line)
+  -- The cite key is not required to share a line with "@type{" (this is how
+  -- OpenReview exports its BibTeX). When parse_type could not find it, pick
+  -- it up from the first subsequent line that supplies it.
+  if self.current.cite_key == nil then
+    local key = vim.fn['matchlist'](line, [[\v^\s*([^, ]+)\s*,\s*(.*)]])
+    if self.empty(key) then
+      self.current.body = self.current.body .. line
+      return false
+    end
+    self.current.cite_key = key[2]
+    line = key[3]
+  end
+
   self.current.level = self.current.level + vim.fn['cmp_vimtex#count'](line, '{') - vim.fn['cmp_vimtex#count'](line, '}')
   if self.current.level > 0 then
     self.current.body = self.current.body .. line
@@ -329,6 +342,10 @@ parser.empty = function(list)
       return false
     end
   end
+
+  -- nil (e.g. an out-of-range matchlist() result) counts as empty; without
+  -- this the guards above silently fall through on a nil argument.
+  return true
 end
 
 return parser