quoted-string grammar seems inconsistent with official parser behavior for empty string

### Title

Grammar for `quoted-string` excludes empty string, but official parser allows `""`

---

### Description

In the JMESPath documentation, the grammar for `quoted-string` is currently defined as:

```ebnf
quoted-string = quote 1*(unescaped-char / escaped-char) quote
```

This definition requires at least one character between the quotes, which formally excludes the empty string `""`.

However, this appears to be inconsistent with the actual behavior of the official JMESPath parser.

---

### Actual Behavior

The official JMESPath implementation (including the online interpreter on [https://jmespath.org/](https://jmespath.org/)) allows empty strings as object keys.

For example, given the following JSON input:

```json
{
"": 123
}
```

The expression:

```jmespath
""
```

correctly evaluates to:

```json
123
```

This behavior is also consistent with JSON semantics, where object keys may be empty strings.

---

### Expected / Documented Behavior

Based on the current grammar definition using `1*`, the empty string `""` would not be considered a valid `quoted-string`, which does not reflect the actual parser behavior.

---

### Suggested Change

To better align the documented grammar with the existing behavior and JSON string semantics, it may be more accurate to define `quoted-string` as:

```ebnf
quoted-string = quote *(unescaped-char / escaped-char) quote
```

This allows zero or more characters between the quotes and explicitly includes the empty string `""`.

---

### Notes

* This issue does **not** propose any change to parser behavior.
* The suggestion is intended purely as a documentation / grammar clarification to reflect existing, widely implemented behavior.