Fix cursor_coordinate going out of bounds into header for empty tables

[Jah-yee]

When a DataTable is empty (no rows or columns), pressing arrow keys could cause cursor_coordinate to become (-1, 0), which corresponds to the header row. This caused the header cell to be highlighted as if it were a selected data cell.

The issue was in _clamp_cursor_coordinate:

row = clamp(row, 0, self.row_count - 1)

When row_count = 0, this becomes clamp(-1, 0, -1), which returns -1, causing the issue.

Fix: Ensure the maximum bound is never negative:

row = clamp(row, 0, max(0, self.row_count - 1))
column = clamp(column, 0, max(0, len(self.columns) - 1))

Fixes #6411