Fix truncateToSeconds extension function signature to receive non-null only

[FrancescoJo]

AS-IS

fun Instant?.truncateToSeconds(): Instant? =
    this?.truncatedTo(ChronoUnit.SECONDS)

TO-BE

fun Instant.truncateToSeconds(): Instant =
    this.truncatedTo(ChronoUnit.SECONDS)

MOTIVATION

Receiving a nullable Instant instance in this case is almost useless. Since Kotlin natively supports nullsafe operator(?.), invoking truncateToSeconds via null safe call still conserve nullity of receiving object. For non-null cases, however, this call converts receiving instance as 'maybe' type and it adds a mandatory, cumbersome non-null assertion operator(!!) which degrades code readibility.

IMPACT

truncateToSeconds calls on nullable instance will not compile after this fix.