Add test to make sure --set flag interprets `null` as `nil`

pkg/strvals/parser_test.go

@@ -131,6 +131,11 @@ func TestParseSet(t *testing.T) {
 			str:    "boolean=true",
 			expect: map[string]interface{}{"boolean": true},
 		},
+		{
+			str:    "is_null=null",
+			expect: map[string]interface{}{"is_null": nil},
+			err:    false,
+		},
 		{
 			str: "name1,name2=",
 			err: true,