Unit tests: optional and string fields

[lealobanov]

Description

Adds unit tests for:
com.nftco.flow.sdk.cadence.OptionalField;
com.nftco.flow.sdk.cadence.StringField;

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[github-actions]

Unit Test Results

  21 files  ±0    21 suites  ±0   27s ⏱️ ±0s
127 tests ±0  126 ✔️ ±0  0 💤 ±0  1 ❌ ±0 

For more details on these failures, see this check.

Results for commit e9f0400. ± Comparison against base commit e9f0400.

♻️ This comment has been updated with latest results.