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)

src/evalfunc.c

@@ -1021,7 +1021,7 @@ static funcentry_T global_functions[] =
     {"getcwd",		0, 2, FEARG_1,	    NULL,
 			ret_string,	    f_getcwd},
     {"getenv",		1, 1, FEARG_1,	    NULL,
-			ret_string,	    f_getenv},
+			ret_any,	    f_getenv},
     {"getfontname",	0, 1, 0,	    NULL,
 			ret_string,	    f_getfontname},
     {"getfperm",	1, 1, FEARG_1,	    NULL,

src/testdir/test_vim9_builtin.vim

@@ -472,6 +472,19 @@ def Test_getchar()
   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)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2583,
 /**/
     2582,
 /**/