time: fix TimeFormatter returning nil for the Unix epoch

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
TimeFormatter caches the last two formatted timestamps in two slots, and an empty slot was marked by its key being 0. But 0 is a valid timestamp, so the epoch matched an empty slot and its nil string was returned as a cache hit:

Fluent::TimeFormatter.new(nil, false, nil).format(0)  #=> nil

format_with_subsec compares slots with EventTime.eq?, which falls back to EventTime#== and compares seconds alone, so there the whole first second of the epoch is affected:

fmt = Fluent::TimeFormatter.new("%Y%m%d %H%M%S.%N", false, nil)
fmt.format(Fluent::EventTime.new(0, 123456789))  #=> nil

The fix is to mark an empty slot by its cached string being nil rather than by its key.
format_nocache always returns a String, so a filled slot can never be mistaken for an empty one. Every other timestamp behaves exactly as before, since key 0 could never have matched it.

Impact is small, and I don't think this is urgent.
A slot keeps the key 0 only until it is written, so a formatter self-heals once it has formatted two distinct timestamps, within a second of the first record.
The exception is a stream whose timestamps are all the epoch: the early return never fills a slot, and out_file then writes every line with an empty time column.
That is reachable via time_type unixtime, which parses with value.to_i and turns an empty or non-numeric time_key into EventTime.new(0), but such a pipeline is misconfigured anyway. The reason to fix it is simply that nil is never a correct result.

Docs Changes:

Release Note:

• time: fix TimeFormatter returning nil for the Unix epoch