public final String getNullableAnnotation() {
      return nullableAnnotation.orElse("");
    }
Returns the string to use as a method annotation to indicate the nullability of this
property. It is either the empty string, if the property is not nullable, or an annotation
string with a trailing space, such as {@code "@`javax.annotation.Nullable` "}, where the
{@code ``} is the encoding used by {@link TypeEncoder}. If the property is nullable by virtue
of its <i>type</i> rather than its method being {@code @Nullable}, this method returns the
empty string, because the {@code @Nullable} will appear when the type is spelled out. In this
case, {@link #nullableAnnotation} is present but empty.
value/src/main/java/com/google/auto/value/processor/AutoValueishProcessor.java