refactor: drop redundant reference check in Labels.isEmpty()

[david-mollitor-db]

What

Simplify Labels.isEmpty() by dropping the redundant this == EMPTY reference check.

Why

Labels.equals() already begins with if (this == o) return true;, so this.equals(EMPTY) already returns true for the EMPTY singleton that the this == EMPTY clause guards. Removing the clause is therefore behavior-preserving, and it was the sole reason for the @SuppressWarnings("ReferenceEquality") annotation, which is removed as well.

// before
@SuppressWarnings("ReferenceEquality")
public boolean isEmpty() {
  return this == EMPTY || this.equals(EMPTY);
}

// after
public boolean isEmpty() {
  return this.equals(EMPTY);
}

This also aligns isEmpty() with the rest of Labels, which already uses this.equals(EMPTY) (e.g. in toString()).

This pull request and its description were written by Isaac.