supermercado (fio collect) is giving warning and generating empty file

**cat aoi.geojson | supermercado burn 19 | mercantile shapes | fio collect > aoi_z19.geojson**

Command running with error but an empty file (aoi_z19.geojson) is generated
.............................................................................................................................................................................................
**The warning message is as following:**

Traceback (most recent call last):
File "c:\programdata\anaconda3\lib\runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "c:\programdata\anaconda3\lib\runpy.py", line 85, in _run_code
exec(code, run_globals)
File "C:\ProgramData\Anaconda3\Scripts\fio.exe\__main__.py", line 9, in
File "c:\programdata\anaconda3\lib\site-packages\click\core.py", line 764, in __call__
return self.main(*args, **kwargs)
File "c:\programdata\anaconda3\lib\site-packages\click\core.py", line 717, in main
rv = self.invoke(ctx)
File "c:\programdata\anaconda3\lib\site-packages\click\core.py", line 1137, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "c:\programdata\anaconda3\lib\site-packages\click\core.py", line 956, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "c:\programdata\anaconda3\lib\site-packages\click\core.py", line 555, in invoke
return callback(*args, **kwargs)
File "c:\programdata\anaconda3\lib\site-packages\click\decorators.py", line 17, in new_func
return f(get_current_context(), *args, **kwargs)
File "c:\programdata\anaconda3\lib\site-packages\fiona\fio\__init__.py", line 18, in wrapper
return f(*args, **kwds)
File "c:\programdata\anaconda3\lib\site-packages\fiona\fio\collect.py", line 58, in collect
first_line = next(stdin)
StopIteration

Reproduce the shown supermercado-to-fio collect command and start at fiona/fio/collect.py line 58, where next(stdin) raises StopIteration. Trace why the pipeline supplies no readable feature and verify that the command produces a non-empty aoi_z19.geojson without the warning.

Written by the indexing model from the issue text.