empty string is converted to empty object in LoopBack3

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## Steps to reproduce

LookBack version: 3.28.0
loopback-connector-mysql version: 7.0.5

This bug occurred after we upgraded the version of loopback-connector-mysql from 4.1.0 to 7.0.5.

define a nullable property in lb3 model like this:

```
"properties": {
"id": {
"type": "string",
"required": false,
"length": 255,
"precision": null,
"scale": null,
"default":null,
"mysql": {
"columnName": "id",
"dataType": "VARCHAR",
"dataLength": 255,
"dataPrecision": null,
"dataScale": null,
"nullable": "Y"
}
}
}
```

And then simply get this model from lb.

## Bbserved Result

If id is an empty string, it will be converted to empty object. If id is null or non-empty string, it will be returned directly.

## Expected Behavior

Empty string shouldn't be converted to empty object, and should be returned directly as null or non-empty string.

## Additional information

node -e 'console.log(process.platform, process.arch, process.versions.node)'
> darwin arm64 20.9.0

npm ls --prod --depth 0 | grep loopback
> npm WARN config production Use `--omit=dev` instead.
├── loopback-boot@3.3.1
├── loopback-component-explorer@6.5.1
├── loopback-component-storage@3.7.0
├── loopback-connector-mongodb@5.6.0
├── loopback-connector-mysql@7.0.5
├── loopback-connector-rest@2.1.0
├── loopback-filters@1.1.1
├── loopback-i18n@1.1.4
├── loopback@3.28.0

Start by reproducing the behavior with LoopBack 3.28.0, loopback-connector-mysql 7.0.5, and the nullable string id model described here. Trace the MySQL connector path used when reading that property; done means an empty string remains an empty string while null and non-empty strings retain their existing behavior.

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