Don't construct needless default value in general case.

There's a check that a yson struct member can be omitted during
encoding. For this check a default value of that member could be
constructed, which might be CPU expensive.

In this patch the default value is not constructed in the case when
is's definitely not used.

This gives improvement from:

```
BenchmarkEncode<TDeepStructLite>       41765 ns        41761 ns        16616 items_per_second=23.9456k/s
```

To:

```
BenchmarkEncode<TDeepStructLite>        8893 ns         8892 ns        78840 items_per_second=112.467k/s
```

Part 2 of <https://nda.ya.ru/t/E9Ji-1bm7Gwv6c>
commit_hash:cd74eb71e07b6e011f65f802d017bdd2b7fe7719

0 files changed, 0 insertions, 0 deletions