Returns an {@code Nullable} describing the given value, if
non-{@code null}, otherwise returns an empty {@code Nullable}.
@param value the possibly-{@code null} value to describe
@param !U the type of the value
@return an {@code Nullable} with a present value if the specified value
is non-{@code null}, otherwise an empty {@code Nullable}
Returns an {@code Nullable} describing the given value, if non-{@code null}, otherwise returns an empty {@code Nullable}.
@param value the possibly-{@code null} value to describe @param !U the type of the value @return an {@code Nullable} with a present value if the specified value is non-{@code null}, otherwise an empty {@code Nullable}