Convert a Roman numeral string to its integer value. Roman numerals use seven symbols: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Normally, symbols are arranged from largest to smallest, but when a smaller symbol appears before a larger one, subtract the smaller value (e.g., IV=4, IX=9, XL=40, XC=90, CD=400, CM=900). Return null for null input and 0 for empty string.
Examples:
Input: roman = 'III'
Output: 3
Explanation: I + I + I = 1 + 1 + 1 = 3
Input: roman = 'LVIII'
Output: 58
Explanation: L + V + I + I + I = 50 + 5 + 1 + 1 + 1 = 58
Input: roman = 'MCMXCIV'
Output: 1994
Explanation: M + CM + XC + IV = 1000 + 900 + 90 + 4 = 1994
Input: roman = null
Output: null
Explanation: Null input returns null
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