Fix ls with detail=False when do not support supports_empty_directories

[LuQQiu]

The given dummy in the test is a string type
while the ls() by default is detail=True, which provides a dictionary instead of a string.
This line throws AssertionError when supports_empty_directories=False and the underlying filesystem does not overwrite the default detail to be False

[martindurant]

Did you find a failing case without this fix?

[LuQQiu]

@martindurant

It's interesting that this code path is not in the existing tests that i could find.
Two conditions are needed:

1. supports_empty_directories = False

@pytest.fixture
    def supports_empty_directories(self):
        return False

2. Ls with default detail=True
def ls(self, path, detail=True, **kwargs):

Tests like local or memory filesystem test do not need to overwrite supports_empty_directories
Tests like S3/Azure filesystem test set the detail=False by default in ls related methods https://github.com/fsspec/s3fs/blob/34a32198188164fd48d4d1abcb267f033d1d1ce1/s3fs/core.py#L966

So my hyphothesis is this line of code never got executed

In my alluxiofs tests which tests against S3 without Alluxio, it will trigger this path because alluxiofs by default ls(detailed=True) following https://github.com/fsspec/filesystem_spec/blob/0c909ee6641381fd8913729b8d803a13a985e5cb/fsspec/spec.py#L313C4-L313C47

[martindurant]

(please merge from master to get tests to run - there was a dependencies versions issue)