patch 8.2.2583: Vim9: cannot compare result of getenv() with null

Problem:    Vim9: cannot compare result of getenv() with null.
Solution:   Make the return type of getenv() "any". (closes #7943)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 3ee8a62..8ec3db1 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -472,6 +472,19 @@
   getchar(true)->assert_equal(0)
 enddef
 
+def Test_getenv()
+  if getenv('does-not_exist') == ''
+    assert_report('getenv() should return null')
+  endif
+  if getenv('does-not_exist') == null
+  else
+    assert_report('getenv() should return null')
+  endif
+  $SOMEENVVAR = 'some'
+  assert_equal('some', getenv('SOMEENVVAR'))
+  unlet $SOMEENVVAR
+enddef
+
 def Test_getcompletion()
   set wildignore=*.vim,*~
   var l = getcompletion('run', 'file', true)