labels field of the annotations got dropped or mangled

**Describe the bug**
Somehow the labels field of the annotations got dropped. I noticed the problem when running the attached JSON file export ([_bad-annotation.json.txt](https://github.com/heartexlabs/label-studio/files/7333787/_bad-annotation.json.txt)) through the CONLL2003 converter.

Here's the problematic section from the JSON file:
```
{
"value" : {
"end" : 2180,
"text" : " ^identified i ",
"start" : 2145
},
"from_name" : "label",
"type" : "labels",
"id" : "ZNgZKDPsmk",
"to_name" : "text"
}
```

By the way, a related problem was that an invalid label got used in the annotations (e.g., b instead of block):
```
{
"value" : {
"start" : 2145,
"end" : 2180,
"text" : " ^identified i\n ",
"labels" : [
"b"
]
},
"type" : "labels",
"from_name" : "label",
"to_name" : "text"
}
```

**To Reproduce**
I'm not sure how to reproduce the problem: I just noticed the problem downstream when doing the conversion. See https://github.com/heartexlabs/label-studio-converter/issues/61#issue-1024620914.

**Environment (please complete the following information):**
- OS: Ubuntu 18.04
- Label Studio Version 1.2

**Additional context**

I created two separate tickets because there are two issues:
1. Task DB getting corrupted
2. Converted not handling annotation spans with missing fields.