Extraneous error with an empty string is reported when `singleError` option is turned on

### Package Versions:

Tested the following combinations:

- `ajv@7.0.3` with `ajv-errors@2.0.0`
- `ajv@6.12.6` with `ajv-errors@1.0.1`

### Background

Good day, I have observed a strange behavior when using `ajv-errors` with `singleError` flag turned on. Honestly, I can't remember why even it was turned on in my code but when I turn it off, this unexpected behavior stops.

The code below is the minimum reproducible example for the issue I am describing. I had a fairly complex schema but I have simplified to the most basic example I could to illustrate the issue.

When I have two `errorMessage` properties, set as follows - one in the top level and the other in one of the fields, an extraneous empty error is reported.

(In my case there were a lot more).

I hope you'd shed some light on this unexpected behavior. Many thanks!

### Code

```javascript
const AJV = require('ajv').default;
const ajvErrors = require('ajv-errors');

const validator = new AJV({ allErrors: true});

// Add AJV errors with singleError: true
ajvErrors(validator, {
singleError:true
});

validator.addSchema({
$id: 'pizzaOrder',
type:'object',
additionalProperties: false,
errorMessage:{
additionalProperties:'Pizza order is invalid'
},
properties:{
price:{
type:'number'
},
toppings:{
type:'object',
additionalProperties: false,
errorMessage: {
_: 'Unsupported topping'
},
properties: {
pineapple:{
type: 'boolean'
}
}
}
}
});

validator.validate('pizzaOrder', {
somethingBad: true,
toppings:{
pineapple:true
}
});

console.log(JSON.stringify(validator.errors, null, 2));;
```

### Expected Result:

Since I have deliberately put an `additionalProperty` called `somethingBad` I would expect it to be reported as follows:

```json
[
{
"keyword": "errorMessage",
"dataPath": "",
"schemaPath": "#/errorMessage",
"params": {
"errors": [
{
"keyword": "additionalProperties",
"dataPath": "",
"schemaPath": "#/additionalProperties",
"params": {
"additionalProperty": "somethingBad"
},
"message": "should NOT have additional properties",
"emUsed": true
}
]
},
"message": "Pizza order is invalid"
}
]
```

### Actual Result:

But actually, as highlighted below, there's an extra error that creeps in with an empty message and empty `params.errors` for `toppings` which actually is totally valid.

```diff
[
+ {
+ "keyword": "errorMessage",
+ "dataPath": "/toppings",
+ "schemaPath": "#/properties/toppings/errorMessage",
+ "params": {
+ "errors": []
+ },
+ "message": ""
+ },
{
"keyword": "errorMessage",
"dataPath": "",
"schemaPath": "#/errorMessage",
"params": {
"errors": [
{
"keyword": "additionalProperties",
"dataPath": "",
"schemaPath": "#/additionalProperties",
"params": {
"additionalProperty": "somethingBad"
},
"message": "should NOT have additional properties",
"emUsed": true
}
]
},
"message": "Pizza order is invalid"
}
]
```

### Temporal Workarounds:

Currently I have turned of `singleError` completely, hopefully it won't affect my other schema's error reports

```diff
ajvErrors(validator, {
- singleError:true // removed
});
```

Another thing I have noticed, which may aid finding the issue is that when I did changed the schema this way:
```diff
{
$id: 'pizzaOrder',
type:'object',
additionalProperties: false,
errorMessage:{
additionalProperties:'Pizza order is invalid'
},
properties:{
price:{
type:'number'
},
toppings:{
type:'object',
additionalProperties: false,
- errorMessage: {
- _: 'Unsupported topping'
- },
+ errorMessage: 'Unsupported topping'
properties: {
pineapple:{
type: 'boolean'
}
}
}
}
}
```

The problem went away too. In my case it's not possible however, since I have other error keyword in my full example.

Let me know if any clarifications are required