Fix deprecated functionality when merging DataFrames

When running the code with Pandas 2.1.1 installad a warning is shown:

memory-channels-processor/src/memory/channels/processor.py:156: FutureWarning:
The behavior of DataFrame concatenation with empty or all-NA entries is deprecated.
In a future version, this will no longer exclude empty or all-NA columns when
determining the result dtypes. To retain the old behavior, exclude the relevant
entries before the concat operation.
  data = pd.concat(data_frames)

Fix the relevant code to be compliant with the changed behaviour...

Possible fixes mentioned also in https://stackoverflow.com/questions/77254777/alternative-to-concat-of-empty-dataframe-now-that-it-is-being-deprecated