handle edge cases in balanced_truncate

[LakminiSenanayake]

#327

The updated version includes the following improvements:

• Safe access to list elements - Ensures parts is non-empty before checking parts[0] == 1.
• Zero-length handling - Adds a check to avoid division by zero when total_shortenable_len == 0.
• Safe truncation - Uses max(..., 0) to prevent negative slicing.
• Cleaner output - Filters out empty parts after truncation to produce a cleaner, more usable output.