[BUG][csharp] [csharp-netcore] producing invalid Dictionary

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

openapi-generator version

5.1.1
and
5.2.0-20210607.065944-108

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

      "example": {
        "type": "object",
        "additionalProperties": true
      }

or additionalProperties: {}

Create a spec with the model above and use csharp or csharp-netcore to generate a client. It errors with error CS0305: Using the generic type 'Dictionary<TKey, TValue>' requires 2 type arguments

The property in code shows as Dictionary without any additional type information

Start by generating a client with the minimal OpenAPI model containing an object with additionalProperties: true or {} using csharp and csharp-netcore. Inspect the generated property that is emitted as Dictionary, reproduce the CS0305 compiler error, and confirm that the generated code uses a valid two-argument Dictionary type.