Make signal provider compatible for teleoperation data

[LoreMoretti]

This PR fixes the following error:

Traceback (most recent call last):
  File "/home/lmoretti/repos/gbmono/install/lib/python3.12/site-packages/robot_log_visualizer/ui/gui.py", line 880, in open_mat_file
    self.__load_mat_file(file_name)
  File "/home/lmoretti/repos/gbmono/install/lib/python3.12/site-packages/robot_log_visualizer/ui/gui.py", line 780, in __load_mat_file
    self.signal_provider.open(file_name)
  File "/home/lmoretti/repos/gbmono/install/lib/python3.12/site-packages/robot_log_visualizer/signal_provider/matfile_signal_provider.py", line 124, in open
    if "log" in root_variable.keys():
                ^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'keys'

when loading data collected from teleoperation.