Add StateHelper#computeIfAbsent

[tandraschko]

Similar to: eclipse-ee4j/mojarra#4568

we already have a eval() with a lambda to provide the default value. However, this defaultValueSupplier does NOT store it as default in the state/stateHelper.

computeIfAbsent however will store the defaultValue in the state.

See PF code:

    /**
     * Tries to retrieve value from stateHelper by key first. If the value is not present (or is null),
     * then it is retrieved from defaultValueSupplier.
     *
     * Should be removed when {@link StateHelper} is extended with similar functionality.
     * (see https://github.com/eclipse-ee4j/mojarra/issues/4568 for details)
     * @param stateHelper The stateHelper to try to retrieve value from
     * @param key The key under which value is stored in the stateHelper
     * @param defaultValueSupplier The object, from which default value is retrieved
     * @param <T> the expected type of returned value
     * @return value from stateHelper or defaultValueSupplier
     */
    public static <T> T computeIfAbsent(StateHelper stateHelper, Serializable key, Supplier<T> defaultValueSupplier) {
        T value = (T) stateHelper.get(key);
        if (value == null) {
            value = defaultValueSupplier.get();
            stateHelper.put(key, value);
        }
        return value;
    }

[tandraschko]

apache/myfaces@65dcf36