Fix KeyError in MountPoint.__repr__()

[CarstenGrohmann]

MountPoint.__repr__() triggered a KeyError exception if the mount point doesn't exist. The commit below fixes this problem.

Solved error:

__________________ test_mountpoint[docker://debian_bookworm] ___________________
[gw1] linux -- Python 3.9.20 /home/runner/work/pytest-testinfra/pytest-testinfra/.tox/py39/bin/python

host = <testinfra.host.Host docker://debian_bookworm>

    def test_mountpoint(host):
        root_mount = host.mount_point("/")
        assert root_mount.exists
        assert repr(root_mount)
        assert isinstance(root_mount.options, list)
        assert "rw" in root_mount.options
        assert root_mount.filesystem
    
        fake_mount = host.mount_point("/fake/mount")
        assert not fake_mount.exists
>       assert repr(fake_mount)

test/test_modules.py:492: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
testinfra/modules/mountpoint.py:108: in __repr__
    self.device,
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <[KeyError('device') raised in repr()] LinuxMountPoint object at 0x7fa72d956b80>

    @property
    def device(self):
        """Return the device associated
    
        >>> host.mount_point("/").device
        '/dev/sda1'
    
        """
>       return self._attrs["device"]
E       KeyError: 'device'

testinfra/modules/mountpoint.py:70: KeyError