fix(validator): session signatures storage

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 43.00%. Comparing base (84556e8) to head (62f748a).

Files with missing lines	Patch %	Lines
collator/src/validator/impls/std_impl/session.rs	92.85%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #428 +/- ## ========================================== - Coverage 43.03% 43.00% -0.03% ========================================== Files 262 262 Lines 45556 45554 -2 Branches 45556 45554 -2 ========================================== - Hits 19603 19591 -12 - Misses 24898 24905 +7 - Partials 1055 1058 +3 ```

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