decodeStyledParameter has an inconsistency for arrays in query

[TheSadlig]

Hi,

I noticed some inconsistencies while using arrays in query.

When the schema is an array and;

1. The query is empty, the function returns nil
2. The query is not empty, the function returns []interface{nil}

This can be reproduced with the following test cases (to add in https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder_test.go#L859 ) :

{
	name:  "no param, for arrays",
	param: &openapi3.Parameter{Name: "something", In: "query", Schema: stringArraySchema},
	query: "",
	want:  nil,
	found: false,
	err:   nil,
},
{
	name:  "missing param, for arrays",
	param: &openapi3.Parameter{Name: "something", In: "query", Schema: stringArraySchema},
	query: "foo=bar",
	want:  nil, // Will fail
	found: false,
	err:   nil,
},

This is due to the first case returning nil via this condition: https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L248
While the second case returns nil via this condition https://github.com/getkin/kin-openapi/blob/master/openapi3filter/req_resp_decoder.go#L524 (that function returns a []any, thus the nil is wrapped in a slice

[TheSadlig]

See #990 for a potential fix