awk: add a test that $NF is empty

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/awk.tests

@@ -406,4 +406,11 @@ testing 'awk FS regex which can match empty string' \
 	'' \
 	'foo--bar'
 
+# last+1 field should be empty (had a bug where it wasn't)
+testing 'awk $NF is empty' \
+	"awk -F '=+' '{print \$NF}'" \
+	"\n" \
+	'' \
+	'a=====123='
+
 exit $FAILCOUNT