Fix #2: preserve zero custom position values

[hahacium]

Summary

Fixes #2.

The custom position object logic previously used truthiness checks for top, bottom, left, and right. That caused valid numeric 0 values to be treated as missing and replaced with auto.

Fix

• Replaced truthiness checks with explicit typeof ... !== 'undefined' checks.
• Preserves valid falsy values such as 0 while still defaulting omitted properties to auto.

Verification

• Used systematic debugging to locate the issue in src/jquery.tips.js inside the position() method.
• Verified the branch diff against master; the final file diff only changes the four position-property checks.
• Automated test/build/lint could not be run because this repository does not contain a package manifest such as package.json or an available test command.

Risk

Low. The change is limited to custom object-based position handling and preserves the existing fallback behavior for undefined properties.