```ts
export function runLengthEncode(input: string): string {
  if (typeof input !== "string") throw new Error("Input must be a string");

  const encoded = [];
  let count = 1;

  for (let i = 0; i < input.length; i++) {
    if (input[i] === input[i + 1]) {
      count++;
    } else {
      if (count > 9) throw new Error("Not supported: run longer than nine characters");
      encoded.push(input[i], count.toString());
      count = 1;
    }
  }

  return encoded.join("");
}

// Example usage:
console.log(runLengthEncode("aaabbc")); // Output: "a3b2c1"
console.log(runLengthEncode("abc"));   // Output: "a1b1c1"
console.log(runLengthEncode(""));      // Output: ""
```