Use computeIfAbsent in CommandLineArgs.addOptionArg

[junhyung8795]

Replace the manual containsKey/put/get three-step pattern in
CommandLineArgs.addOptionArg with Map.computeIfAbsent, which expresses
the same intent more concisely and avoids the redundant second map lookup.

Before:

if (!this.optionArgs.containsKey(optionName)) {
    this.optionArgs.put(optionName, new ArrayList<>());
}
if (optionValue != null) {
    this.optionArgs.get(optionName).add(optionValue);
}

After:

List<String> values = this.optionArgs.computeIfAbsent(optionName, key -> new ArrayList<>());
if (optionValue != null) {
    values.add(optionValue);
}

Behavior is identical: the option name is always registered in the map (even
without a value), and the value is added only when non-null.

[bclozel]

Please sign the DCO.