Fix single fixation bug and entropy NaN handling

[oscararenas12]

Bug Fixes: Fixation Filtering & Entropy NaN Handling

Fix 1: Single Fixation Not Saved

File: DataFilter.java
Problem: When a dataset has only one fixation, zero fixations were saved.
Cause: Edge case check required filtered.rowCount() != 0, which fails for single fixation.
Solution: Check lastValidFixation != null instead.

Before (1 fixation):

• Row 1: fixation 1 → nothing saved
• End of file → check fails (rowCount == 0)

After (1 fixation):

• Row 1: fixation 1 → nothing saved
• End of file → lastValidFixation exists, save it

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Fix 2: Entropy Returns NaN Instead of 0

File: GazeEntropy.java
Problem: stationary_entropy and transition_entropy returned 0.0 when data was empty or invalid.
Solution: Return NaN when:

• aoiProbability is empty (no fixations)
• transitionProbability is empty (no transitions)

Before (empty data):

• stationary_entropy: 0.0

After (empty data):

• stationary_entropy: NaN
• transition_entropy: NaN