Convert an integer from 1 to 3999 to a Roman numeral. Roman numerals use combinations of seven symbols: I (1), V (5), X (10), L (50), C (100), D (500), and M (1000). Subtractive notation is used in specific cases: IV (4), IX (9), XL (40), XC (90), CD (400), and CM (900). Return null for invalid inputs (null or numbers outside the 1-3999 range).
Examples:
Input: num = 3
Output: "III"
Explanation: 3 is represented as three I symbols
Input: num = 58
Output: "LVIII"
Explanation: L = 50, V = 5, III = 3
Input: num = 1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90, and IV = 4
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