OutputMethod.LOGGER without logging_logger appends None to handlers, crashes on flush()

Description

When output_methods=[OutputMethod.LOGGER] is passed without providing a logging_logger, None is appended to the output handlers list. This causes AttributeError: 'NoneType' object has no attribute 'out' on flush(), and silently swallows all output (including CSV) on stop() because @suppress(Exception) catches the error.

Reproduction

from codecarbon import EmissionsTracker
from codecarbon.output_methods.base_output import OutputMethod

tracker = EmissionsTracker(output_methods=[OutputMethod.LOGGER])
tracker.start()
tracker.flush()  # AttributeError: 'NoneType' object has no attribute 'out'

With stop() instead of flush(), the crash is silently swallowed by @suppress(Exception) — no error is raised, but ALL output handlers (including CSV) are skipped because the exception aborts the handler loop.

Root cause

In emissions_tracker.py line 638-639, _init_output_methods() does:

if OutputMethod.LOGGER in methods:
    self._output_handlers.append(self._logging_logger)

When no logging_logger is provided, self._logging_logger is None (set by _set_from_conf with no default). So None gets appended to _output_handlers. Later, iterating over handlers and calling .out() on None crashes.

Expected behavior

Either:

• Validate that self._logging_logger is not None before appending, and log a warning / raise an error if LOGGER method was requested without a logger
• Or skip the LOGGER method with a warning