[output-stream] Accept a NULL filename

Principally to support creating a dummy vector surface (i.e.
cairo_ps_surface_create (NULL, 1, 1)) that can be used to determine font
extents (or target font options) before opening an output file, but also
because we currently fail to do any sanity checking at the entry point.

src/cairo-output-stream.c

@@ -583,6 +583,9 @@ _cairo_output_stream_create_for_filename (const char *filename)
     stdio_stream_t *stream;
     FILE *file;
 
+    if (filename == NULL)
+	return _cairo_null_stream_create ();
+
     file = fopen (filename, "wb");
     if (file == NULL) {
 	switch (errno) {