Add nil checks for name and file in sortDoc

`name` and `file` don't appear to be guaranteed non-nil; because of this, if there are cases where the key is present on one element but not the other, the equality check will fall through to the  less-than check and this results in a crash comparing `string` against `nil`.

Resolves: #3111

Author: dljsjr

script/cli/doc/export.lua

@@ -72,11 +72,11 @@ function export.positionOf(rowcol)
 end
 
 function export.sortDoc(a,b)
-    if a.name ~= b.name then
+    if a.name and b.name and a.name ~= b.name then
         return a.name < b.name
     end
 
-    if a.file ~= b.file then
+    if a.file and b.file and a.file ~= b.file then
         return a.file < b.file
     end