Inconsistent behavior when compiling a schema with additionalProperties

[rlouf]

The behavior of the compiler when additionalProperties is set is inconsistent. For instance the following will not compile:

import json
from outlines_core.fsm.json_schema import build_regex_from_schema


schema = {
    "$schema": "http://json-schema.org/draft-04/schema#",
    "id": "#languagemap",
    "type": "object",
    "patternProperties": {
        "^(yes|no)$": {
        "type": "string"
        }
    },
    "additionalProperties": False
}

regex_str = build_regex_from_schema(json.dumps(schema))
# ValueError: Unsupported JSON Schema structure false 
# Make sure it is valid to the JSON Schema specification and check if it's supported by Outlines.
# If it should be supported, please open an issue.

But it compiles when setting additionalProperties to True:

```python
import json
from outlines_core.fsm.json_schema import build_regex_from_schema


schema = {
    "$schema": "http://json-schema.org/draft-04/schema#",
    "id": "#languagemap",
    "type": "object",
    "patternProperties": {
        "^(yes|no)$": {
        "type": "string"
        }
    },
    "additionalProperties": True
}

regex_str = build_regex_from_schema(json.dumps(schema))

And compiles when additionalProperties is set to False but we replace patternProperties with regular properties:

import json
from outlines_core.fsm.json_schema import build_regex_from_schema


schema = {
    "$schema": "http://json-schema.org/draft-04/schema#",
    "id": "#languagemap",
    "type": "object",
    "properties": {
        "yes": {
        "type": "string"
        }
    },
    "additionalProperties": False
}

regex_str = build_regex_from_schema(json.dumps(schema))