Fixed null issue with entry operators

pkg/yqlib/doc/Entries.md

@@ -35,6 +35,19 @@ will output
   value: b
 ```
 
+## to_entries null
+Given a sample.yml file of:
+```yaml
+null
+```
+then
+```bash
+yq eval 'to_entries' sample.yml
+```
+will output
+```yaml
+```
+
 ## from_entries map
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/operator_entries.go

@@ -59,9 +59,11 @@ func toEntriesOperator(d *dataTreeNavigator, context Context, expressionNode *Ex
 		case yaml.SequenceNode:
 			results.PushBack(toEntriesfromSeq(candidate))
 		default:
+			if candidateNode.Tag != "!!null" {
 				return Context{}, fmt.Errorf("%v has no keys", candidate.Node.Tag)
 			}
 		}
+	}
 
 	return context.ChildContext(results), nil
 }
@@ -133,26 +135,26 @@ func withEntriesOperator(d *dataTreeNavigator, context Context, expressionNode *
 	//to_entries on the context
 	toEntries, err := toEntriesOperator(d, context, expressionNode)
 	if err != nil {
-		return Context{}, nil
+		return Context{}, err
 	}
 
 	//run expression against entries
 	// splat toEntries and pipe it into Rhs
 	splatted, err := splat(d, toEntries, traversePreferences{})
 	if err != nil {
-		return Context{}, nil
+		return Context{}, err
 	}
 
 	result, err := d.GetMatchingNodes(splatted, expressionNode.Rhs)
 	log.Debug("expressionNode.Rhs %v", expressionNode.Rhs.Operation.OperationType)
 	log.Debug("result %v", result)
 	if err != nil {
-		return Context{}, nil
+		return Context{}, err
 	}
 
 	collected, err := collectOperator(d, result, expressionNode)
 	if err != nil {
-		return Context{}, nil
+		return Context{}, err
 	}
 
 	//from_entries on the result

pkg/yqlib/operator_entries_test.go

@@ -21,6 +21,13 @@ var entriesOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!seq)::- key: 0\n  value: a\n- key: 1\n  value: b\n",
 		},
 	},
+	{
+		description: "to_entries null",
+		document:    `null`,
+		expression:  `to_entries`,
+		expected: []string{
+		},
+	},
 	{
 		description: "from_entries map",
 		document:    `{a: 1, b: 2}`,