Call writePrefix() from ctor in SinkToOutputStream

This will fix lack of headers for the following formats if there are
zero rows:
- TSVWithNames
- CSVWithNames
- TSVWithNamesAndTypes

src/Processors/Sources/SinkToOutputStream.cpp

@@ -8,15 +8,11 @@ SinkToOutputStream::SinkToOutputStream(BlockOutputStreamPtr stream_)
     : ISink(stream_->getHeader())
     , stream(std::move(stream_))
 {
+    stream->writePrefix();
 }
 
 void SinkToOutputStream::consume(Chunk chunk)
 {
-    if (!initialized)
-        stream->writePrefix();
-
-    initialized = true;
-
     stream->write(getPort().getHeader().cloneWithColumns(chunk.detachColumns()));
 }
 

src/Processors/Sources/SinkToOutputStream.h

@@ -22,7 +22,6 @@ protected:
 
 private:
     BlockOutputStreamPtr stream;
-    bool initialized = false;
 };
 
 }