fix(schema): adding undefined to stringType validator

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b64f778) 93.15% compared to head (f231f34) 93.15%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #727 +/- ## ======================================= Coverage 93.15% 93.15% ======================================= Files 95 95 Lines 2660 2660 Branches 640 640 ======================================= Hits 2478 2478 Misses 182 182 ``` | [Impacted Files](https://app.codecov.io/gh/couchbaselabs/node-ottoman/pull/727?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=couchbaselabs) | Coverage Δ | | |---|---|---| | [src/schema/types/string-type.ts](https://app.codecov.io/gh/couchbaselabs/node-ottoman/pull/727?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=couchbaselabs#diff-c3JjL3NjaGVtYS90eXBlcy9zdHJpbmctdHlwZS50cw==) | `97.56% <100.00%> (ø)` | | | [src/utils/setValueByPath.ts](https://app.codecov.io/gh/couchbaselabs/node-ottoman/pull/727?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=couchbaselabs#diff-c3JjL3V0aWxzL3NldFZhbHVlQnlQYXRoLnRz) | `100.00% <100.00%> (ø)` | |

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