patch 8.2.3104: Vim9: unspecified function type causes type error

Problem:    Vim9: unspecified function type causes type error.
Solution:   Don't check type when min_argcount is negative. (issue #8492)
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 92ffa00..5f5b5d7 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -661,6 +661,15 @@
   CheckDefExecAndScriptFailure(lines, 'E1012:', 5)
 enddef
 
+def PartFunc(b: bool): string
+  return 'done'
+enddef
+
+def Test_assignment_partial()
+  var Partial: func(): string = function(PartFunc, [true])
+  assert_equal('done', Partial())
+enddef
+
 def Test_assignment_list_any_index()
    var l: list<number> = [1, 2]
   for  [x, y, _]