Refactor palindrom checking algorithm

[bgdnrvsky]

Description

• Use simpler approach to check if the string is a palindrome
• Add unit test for empty palindrome

Checklist:

• [x] I ran bellow commands using the latest version of rust nightly.
• [x] I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• [x] I ran cargo fmt just before my last commit.
• [x] I ran cargo test just before my last commit and all tests passed.
• [x] I checked COUNTRIBUTING.md and my code follows its guidelines.