Empty tuple type does not generate valid JSON schema

[ghost]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

 python -c "import pydantic.utils; print(pydantic.utils.version_info())"
             pydantic version: 1.8.1
            pydantic compiled: True
                 install path: /Users/smacpher/.virtualenvs/nucleus/lib/python3.6/site-packages/pydantic
               python version: 3.6.9 (default, Oct 16 2019, 11:14:39)  [GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.10.44.4)]
                     platform: Darwin-17.7.0-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']

import jsonschema
import pydantic

from typing import Tuple

class Model(pydantic.BaseModel):
    f: Tuple[()]

jsonschema.Draft7Validator.check_schema(Model.schema())

This fails with:

SchemaError: [] is not valid under any of the given schemas

Failed validating 'anyOf' in metaschema['properties']['properties']['additionalProperties']['properties']['items']:
    {'anyOf': [{'$ref': '#'}, {'$ref': '#/definitions/schemaArray'}],
     'default': True}

On schema['properties']['f']['items']:
    []

The generated schema is:

{'title': 'Model',
 'type': 'object',
 'properties': {'f': {'title': 'F', 'type': 'array', 'items': []}},
 'required': ['f']}

Is this expected behavior? Would the desired behavior be to set maxItems: 0 and remove items: [] like such (that would make the generated schema valid)?

{'title': 'Model',
 'type': 'object',
 'properties': {'f': {'title': 'F', 'type': 'array', 'maxItems': 0}},
 'required': ['f']}

BTW, glad to see that empty tuples (Tuple[()]) are now supported (#2318)! Thank you 🙏 .

[PrettyWood]

Hi @smacpher-myst and thanks a lot for reporting!
I opened #2497 and tackled your issue at the same time. Feedback welcome :)

[ghost]

@PrettyWood no problem! Awesome, thanks for the super quick turn around :D. Looked over your changes and they look good to me – approved as well (FWIW).