Handle z/OS Unix ‘set’ output

Problem and fix reported by Jeff Shimoda in:
https://lists.gnu.org/r/autoconf-patches/2022-06/msg00007.html
* tests/local.at (AT_CONFIG_CMP): Treat ‘var=""’ like ‘var=''’.
Copyright-paperwork-exempt: Yes

tests/local.at

@@ -476,6 +476,7 @@ do
   $SED '/^ac_cv_/ b skip
 	/^m4_defn([m4_re_word])=./ !d
 	/^[[^=]]*='\'''\''$/ d
+	/^[[^=]]*=""$/ d
 	/^a[[cs]]_/ d
 	: skip
 	/^OLDPWD=/ d