Bug 264971

ScriptBufferSourceProvider sometimes receives a ScriptBuffer with 2 data segments where 2nd segment is empty (size 0, buffer null). The 2nd empty segment was likely added in WorkerScriptLoader::didFinishLoading by append(decoder->flush()) where flush can return an empty string.
ScriptBufferSourceProvider will call makeContiguous on scriptBuffer which for the above case end up making an unnecessary copy and this memory is kept alive by ScriptBufferSourceProvider thus increasing memory usage.

ScriptBuffer should not add an empty segment to the buffer when append is called with empty string.