Fix ZeroDivisionError in pointplot when dodge=True and a single hue level exists

[notval98]

This PR fixes a ZeroDivisionError in sns.pointplot that occurs when dodge=True and the dataset contains only a single hue level. The issue arises because step_size is calculated using (n_hue_levels - 1), which results in a division by zero when n_hue_levels == 1.

Changes Made:
Updated categorical.py to prevent division by zero in the dodge calculation by adding a conditional check:
if n_hue_levels > 1:
step_size = dodge / (n_hue_levels - 1)
else:
step_size = 0 # Prevents division by zero when only one hue level exists

This ensures that when there is only one hue level, the dodge offset remains 0, preventing unnecessary adjustments.

Issue Reference:
Fixes #3825

Testing:
Verified that pointplot no longer raises an error when dodge=True with a single hue level.