Improve FirstNonRepeatedChar.java: Add authoritative references, input validation, and more test cases

[Piyush-Kumar62]

What’s improved

• Added references to LeetCode and GeeksforGeeks for further reading and credibility.
• Enhanced comments and documentation for clarity.
• Added robust input validation for null and empty strings.
• Included more diverse test cases (repeated, unique, empty, single-char, null input).
• Preserved O(n) time and space complexity and clean logic.

References

• https://leetcode.com/problems/first-unique-character-in-a-string/description/
• https://www.geeksforgeeks.org/problems/non-repeating-character-1587115620/1

This makes the solution robust, instructive, and ideal for DSA learners and Hacktoberfest contributors.