fix: fixed optional value decoding failure with `HelperCoder` when value doesn't exist

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b615251) 95.39% compared to head (1175c94) 95.05%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #35 +/- ## ========================================== - Coverage 95.39% 95.05% -0.35% ========================================== Files 76 76 Lines 1804 1819 +15 ========================================== + Hits 1721 1729 +8 - Misses 83 90 +7 ``` | [Files](https://app.codecov.io/gh/SwiftyLab/MetaCodable/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SwiftyLab) | Coverage Δ | | |---|---|---| | [...bleMacroPlugin/Variables/HelperCodedVariable.swift](https://app.codecov.io/gh/SwiftyLab/MetaCodable/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SwiftyLab#diff-U291cmNlcy9Db2RhYmxlTWFjcm9QbHVnaW4vVmFyaWFibGVzL0hlbHBlckNvZGVkVmFyaWFibGUuc3dpZnQ=) | `100.00% <100.00%> (ø)` | | | [Sources/MetaCodable/HelperCoders/HelperCoder.swift](https://app.codecov.io/gh/SwiftyLab/MetaCodable/pull/35?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SwiftyLab#diff-U291cmNlcy9NZXRhQ29kYWJsZS9IZWxwZXJDb2RlcnMvSGVscGVyQ29kZXIuc3dpZnQ=) | `75.00% <100.00%> (-1.93%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/SwiftyLab/MetaCodable/pull/35/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SwiftyLab)

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