openapi3filter: Remove inconsistency for arrays in queries

[TheSadlig]

See #989 for details, but in a nutshell:

• DecodeArray returns a []any, which means that whenever DecodeArray returns nil, it will actually return a []interface{nil}
• Viewed from decodeStyledParameter, this is an inconsistent behaviour, since decodeStyledParameter returns nil when the query parameters are empty, AND []interface{nil} when there is at least another parameter set.

I am unsure as to whether this is the best way to fix that issue, and am open to suggestions.

Thanks