Not starting DDB-local when running sls offline

**Actual Behaviour**

Not starting local dynamo when running `sls offline`

**Steps to reproduce it**

```yaml
plugins:
- serverless-plugin-typescript
- serverless-plugin-optimize
- serverless-pseudo-parameters
- serverless-dynamodb-local
- serverless-offline

custom:
serverless-offline:
useChildProcesses: true
dynamodb:
start: # why doesn't this automatically start?
port: 4455
# inMemory: true
migrate: true
stages:
- dev
```

**LogCat for the issue**

Provide logs for the crash here
```
❯ sls offline
Serverless: Running "serverless" installed locally (in service node_modules)
Serverless: Compiling with Typescript...
Serverless: Using local tsconfig.json
Serverless: Typescript compiled.
Serverless: Watching typescript files...
offline: Starting Offline: dev/eu-west-1.
offline: Offline [http for lambda] listening on http://localhost:3002

┌─────────────────────────────────────────────────────────────────────────────────┐
│ │
│ POST | http://localhost:3000/dev...... │ │
└─────────────────────────────────────────────────────────────────────────────────┘

offline: [HTTP] server ready: http://localhost:3000 🚀
offline:
offline: Enter "rp" to replay the last request
```

**Would you like to work on the issue?**

Not sure where to start

Reproduce the issue with the supplied plugin order and configuration by running `sls offline`. Trace how `serverless-dynamodb-local` and `serverless-offline` are initialized, then verify that local DynamoDB starts on port 4455 with migrations when the command completes.

Written by the indexing model from the issue text.