Partial attempt at allowEmptyValue clarification

This attempts to clarify based on objections to the previous attempt
to clarify.  However, it is incomplete as the concept of "empty value"
is not well-defined.

versions/3.0.4.md

@@ -1044,7 +1044,7 @@ Field Name | Type | Description
 <a name="parameterDescription"></a>description | `string` | A brief description of the parameter. This could contain examples of use. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
 <a name="parameterRequired"></a>required | `boolean` | Determines whether this parameter is mandatory. If the [parameter location](#parameterIn) is `"path"`, this property is **REQUIRED** and its value MUST be `true`. Otherwise, the property MAY be included and its default value is `false`.
 <a name="parameterDeprecated"></a> deprecated | `boolean` | Specifies that a parameter is deprecated and SHOULD be transitioned out of usage. Default value is `false`.
-<a name="parameterAllowEmptyValue"></a> allowEmptyValue | `boolean` | Sets the ability to pass empty-valued parameters. This is valid only for `query` parameters and allows sending a parameter with an empty value. Default value is `false`. If [`style`](#parameterStyle) is used, and if behavior is `n/a` (cannot be serialized), the value of `allowEmptyValue` SHALL be ignored. Use of this property is NOT RECOMMENDED, as it is likely to be removed in a later revision.
+<a name="parameterAllowEmptyValue"></a> allowEmptyValue | `boolean` | Sets the ability to pass empty-valued parameters. This is valid only for `query` parameters and allows sending a parameter with an empty value. If `false`, which is the default value, then empty values MUST NOT be accepted. ***TODO: What values are empty?  RFC6570 defines `null`, `[]`, `{}`, and objects with only `null` properties (e.g. `{"foo": null, "bar": null}` to be "undefined" and serialize as empty values, but it does not consider the empty string to be undefined even though it also serializes as an empty value.  Also, how does this interact with `content`, which is not RFC6570-based and may result in empty values through any number of different circumstances.*** If [`style`](#parameterStyle) is used, and if behavior is `n/a` (cannot be serialized), the value of `allowEmptyValue` MUST NOT be interpreted to allow empty values to be serialized.
 
 ###### Fixed Fields for use with `schema`