fix: handle undefined head option in toString()

[ctonneslan]

Summary

Fixes toString() crashing when the Table constructor receives head: undefined.

Problem

const table = new Table({ head: undefined });
table.push(['a', 'b']);
table.toString(); // TypeError: Cannot read properties of undefined (reading 'length')

The head variable on line 89 takes options.head directly without defaulting, so undefined propagates to .length checks on lines 92 and 95.

Fix

Default head to an empty array: options.head || []

This matches the behavior when head is not passed at all (the default options already set head: [] on line 48).

Fixes #178