fix(iter): make pipeline termination conditions consistent (#24614)

If an iterator pipeline stage returns nil as its first return value, the
other return values are ignored and it is treated as if that stage
returned only nil (the semantics of returning nil are different between
different stages). This is consistent with how for loops work in Lua
more generally, where the for loop breaks when the first return value
from the function iterator is nil (see :h for-in for details).

test/functional/lua/iter_spec.lua

@@ -154,6 +154,9 @@ describe('vim.iter', function()
     eq({1, 2}, vim.iter(t):slice(1, 2):totable())
     eq({10}, vim.iter(t):slice(10, 10):totable())
     eq({8, 9, 10}, vim.iter(t):slice(8, 11):totable())
+
+    local it = vim.iter(vim.gsplit('a|b|c|d', '|'))
+    matches('slice%(%) requires a list%-like table', pcall_err(it.slice, it, 1, 3))
   end)
 
   it('nth()', function()
@@ -396,39 +399,4 @@ describe('vim.iter', function()
       { item_3 = 'test' },
     }, output)
   end)
-
-  it('handles nil values', function()
-    local t = {1, 2, 3, 4, 5}
-    do
-      local it = vim.iter(t):enumerate():map(function(i, v)
-        if i % 2 == 0 then
-          return nil, v*v
-        end
-        return v, nil
-      end)
-      eq({
-        { [1] = 1 },
-        { [2] = 4 },
-        { [1] = 3 },
-        { [2] = 16 },
-        { [1] = 5 },
-      }, it:totable())
-    end
-
-    do
-      local it = vim.iter(ipairs(t)):map(function(i, v)
-        if i % 2 == 0 then
-          return nil, v*v
-        end
-        return v, nil
-      end)
-      eq({
-        { [1] = 1 },
-        { [2] = 4 },
-        { [1] = 3 },
-        { [2] = 16 },
-        { [1] = 5 },
-      }, it:totable())
-    end
-  end)
 end)