Fixes AsyncContentsManager#exists

[icankeep]

There is a bug in AsyncContentsManager#exists.

async def exists(self, path):
    return await (ensure_async(self.file_exists(path)) or ensure_async(self.dir_exists(path)))

it always returns False when file_exists returns False

reproduce code:

from jupyter_server.utils import ensure_async
import asyncio


async def dir_exists():
    return True


async def file_exists():
    return False


async def exists():
    return await (ensure_async(file_exists()) or ensure_async(dir_exists()))


if __name__ == '__main__':
    a = asyncio.get_event_loop().run_until_complete(exists())
    print(a)

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[codecov-commenter]

Codecov Report

Merging #542 (186949d) into master (450010e) will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
+ Coverage   76.33%   76.35%   +0.02%     
==========================================
  Files         110      110              
  Lines        9816     9816              
  Branches     1067     1067              
==========================================
+ Hits         7493     7495       +2     
+ Misses       1943     1942       -1     
+ Partials      380      379       -1     

Impacted Files	Coverage Δ
jupyter_server/services/contents/manager.py	87.76% <0.00%> (ø)
jupyter_server/tests/services/kernels/test_api.py	99.01% <0.00%> (+1.96%)	⬆️

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[blink1073]

Thanks!

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