```typescript
/**
 * Encodes a string using run-length encoding.
 *
 * Replaces every maximal run of one repeated character with that character followed by the run length,
 * in order: "aaabbc" encodes to "a3b2c1". A run of length one still carries its count, so "abc" encodes to "a1b1c1".
 *
 * The empty string encodes to the empty string.
 *
 * @param input The input string to be encoded.
 */
export function runLengthEncode(input: string): string {
    if (typeof input !== 'string') {
        throw new Error('Input must be a string');
    }

    const result = [];
    let currentChar = '';
    let charCount = 0;

    for (const char of input) {
        if (char === currentChar) {
            charCount++;
        } else {
            if (currentChar) {
                result.push(currentChar + (charCount > 1 ? charCount : ''));
            }
            currentChar = char;
            charCount = 1;
        }
    }

    if (currentChar) {
        result.push(currentChar + (charCount > 1 ? charCount : ''));
    }

    return result.join('');
}
```