Fixing readonly ops not to modify context when paths dont exist

pkg/yqlib/operator_variables_test.go

@@ -5,6 +5,14 @@ import (
 )
 
 var variableOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:    true,
+		document:   `{}`,
+		expression: `.a.b as $foo`,
+		expected: []string{
+			"D0, P[], (doc)::{}\n",
+		},
+	},
 	{
 		description: "Single value variable",
 		document:    `a: cat`,