exec: support sparse input

After previous commit, `:exec` support sparse output, but not sparse
args. It seems like a good idea to support it too, since it's too easy
now.

Follows #350

CHANGELOG.md

@@ -5,6 +5,7 @@
   * Fixed incorrent unix socket path length check (gh-341).
   * Now net_box_uri can be accepted as table (gh-342).
   * Fixed returning values from `Server:exec()` if some of them are nil (gh-350).
+  * Support `nil` args for `Server:exec()`.
 
 ## 1.0.0
 

luatest/server.lua

@@ -709,7 +709,7 @@ function Server:exec(fn, args, options)
         if args == nil then
             result = {pcall(fn)}
         else
-            result = {pcall(fn, unpack(args))}
+            result = {pcall(fn, unpack_sparse_array(args))}
         end
         if not result[1] then
             if type(result[2]) == 'table' then

test/autorequire_luatest_test.lua

@@ -113,3 +113,10 @@ g.test_exec_with_sparse_output = function()
     t.assert_equals(res1, nil)
     t.assert_equals(res2, 'some error')
 end
+
+g.test_exec_with_sparse_input = function()
+    g.server:exec(function(v1, v2)
+        t.assert_equals(v1, nil)
+        t.assert_equals(v2, 'val')
+    end, {nil, 'val'})
+end