There are several problems with the way JBossWS handles marshalling of arrays that are either empty or null.
Upon response null arrays become empty arrays, or empty arrays with one null element, depending on the encoding style:

DocLitWrapped:

<s:Envelope xmlns:s='http://schemas.xmlsoap.org/soap/envelope/'>
<s:Header/>
<s:Body>
<RetByteArray xmlns='http://tempuri.org/'>
<inByteArray a:nil='true' xmlns:a='http://www.w3.org/2001/XMLSchema-instance'/>
</RetByteArray>
</s:Body>
</s:Envelope>

<env:Envelope xmlns:env='http://schemas.xmlsoap.org/soap/envelope/'>
<env:Header/>
<env:Body>
<ns1:RetByteArrayResponse xmlns:ns1='http://tempuri.org/' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
<ns1:RetByteArrayResult/>
</ns1:RetByteArrayResponse>
</env:Body>
</env:Envelope>

DocLitBare:

<s:Envelope xmlns:s='http://schemas.xmlsoap.org/soap/envelope/'>
<s:Header/>
<s:Body>
<inArrayString1D i:nil='true' xmlns='http://tempuri.org/' xmlns:i='http://www.w3.org/2001/XMLSchema-instance'/>
</s:Body>
</s:Envelope>

<env:Envelope xmlns:env='http://schemas.xmlsoap.org/soap/envelope/'>
<env:Header/>
<env:Body>
<ns1:RetArrayString1DResult xmlns:ns1='http://tempuri.org/' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
<ns_string:string xmlns:ns_string='http://schemas.microsoft.com/2003/10/Serialization/Arrays' xsi:nil='1'/>
</ns1:RetArrayString1DResult>
</env:Body>
</env:Envelope>