questionnaire functions return 0.0 when row entries are NaN --> should return NaN

[victoria1509]

Description
Returning 0.0 instead of NaN in certain questionnaire functions in Biopsykit when the entries (rows) contain only NaN values, particularly when calculating the sum. For instance, NaN is returned as the value when calculating the mean for a subscale/score.

To Reproduce
Steps to reproduce the behavior:

1. Use Biopsykit functions that involve sum calculations for subscales/scores.
2. Provide a dataset with rows containing only NaN values.3.
3. Calculate the score/subscales
4. Check NaN rows in returned df

Expected behavior
When calculating the sum for a subscale/score, the function should return NaN if the participant's row contains only NaN values.

System Specifications:

• Operating System: macOS
• BioPsyKit version: 0.9.0
• Python version: 3.9