fix(iter): ArrayIter:last returns nil when filtered to empty #34697

Problem: After filtering out all elements, ArrayIter:last still returns a stale element.
Solution: Add check for self._head == self._tail and return nil early.

Fix #34696

(cherry picked from commit 4fe51dfdae)

test/functional/lua/iter_spec.lua

@@ -339,6 +339,15 @@ describe('vim.iter', function()
     local s = 'abcdefghijklmnopqrstuvwxyz'
     eq('z', vim.iter(vim.split(s, '')):last())
     eq('z', vim.iter(vim.gsplit(s, '')):last())
+    eq(
+      nil,
+      vim
+        .iter({ 1, 2, 3, 4, 5 })
+        :filter(function()
+          return false
+        end)
+        :last()
+    )
   end)
 
   it('enumerate()', function()