Object with an empty string as key getting handled incorrectly

Consider this object:

```
{
"foo": {
"": {
"bar": {
"baz": 123
}
}
}
```

The following path returns `[123]`, as expected:

```
jmespath.search('foo."".bar.*', test)
[123]
```

However, when I put a wildcard in place of the key which is an empty string, I don't get a result:

```
jmespath.search('foo.*.bar.*', test) is None
True
```
This is strange, because if I don't specify the second wildcard, the first one works and I do get a result:

```
jmespath.search('foo.*.bar', test)
[{'baz': 123}]
```

I've asked on gitter and have been advised this might be a bug and not my misunderstanding of the JMESPath syntax.

I'm on JMESPath version 0.9.4, installed by pip on Python 3.