Add isEmpty default method to CharSequence

Summary

Adding an isEmpty default method to CharSequence would harmonize String with other CharSequence implementations (StringBuilder etc).

Problem

isEmpty is shorter and more concise than length() == 0. Checking for and filtering out empty Strings and other CharSequences is a common occurrence in code, and having an explicit method allows its use as a method reference.

Solution

Add a default method isEmpty() to CharSequence.

Specification

Add the following to java.lang.CharSequence:

/**
 * Returns {@code true} if this character sequence is empty.
 *
 * @implSpec
 * The default implementation returns the result of calling {@code length() == 0}.
 *
 * @return {@code true} if {@link #length()} is {@code 0}, otherwise
 * {@code false}
 *
 * @since 15
 */
default boolean isEmpty() {
    return this.length() == 0;
}

Also add an override with clarifications to java.nio.CharBuffer:

/**
 * Returns {@code true} if this character buffer is empty.
 *
 * @return {@code true} if there are {@code 0} remaining characters,
 *         otherwise {@code false}
 *
 * @since 15
 */
public final boolean isEmpty() {
    return remaining() == 0;
}