Fix empty strings in activity counts

[sawyerb-ksu]

Adds a check for blank strings "" within the session activities objects to correct issues such as:

...
{
    "timestamp": 1758644704,
    "location": 53,
    "activities": [
        62,
        ""
    ],
    "number": 1
},
...

When a blank string is passed in the activities array, it creates a 500 error on the server and the database transaction cannot complete. This change sanitizes/removes blank strings from the data to prevent this error.

Addresses Issue #164