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/evalfunc.c b/src/evalfunc.c
index bee76c6..29bbe8a 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1021,7 +1021,7 @@
     {"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,