fix: getDesignTypeForProperty

[mdbetancourt]

Fixes #6522
after some checks i notice:

class MyModel {
      @propertyDecorator() // with decorator
      prop: string;
}
const meta = MetadataInspector.getDesignTypeForProperty(
      MyModel.prototype,
      'prop',
);
meta // String
---
class MyModel {
      @propertyDecorator()
      prop: string;
}
const meta = MetadataInspector.getDesignTypeForProperty(
      MyModel, // without prototype
      'prop',
);
meta // always undefined
---
class MyModel { 
      prop: string; // without decorator
}
const meta = MetadataInspector.getDesignTypeForProperty(
      MyModel.prototype,
      'prop',
);
meta // always undefined

so https://github.com/strongloop/loopback-next/blob/master/packages/metadata/src/__tests__/unit/inspector.unit.ts#L680-L687 is a useless tests (because always is gonna be undefined)
and https://github.com/strongloop/loopback-next/blob/0396ab058bc90d1a6a3da37f10353f481003301a/packages/metadata/src/inspector.ts#L210-L217 is not true

• DCO (Developer Certificate of Origin) signed in all commits
• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated