fix: change empty to property

[coroa]

In view of #424 (ie. empty being mostly True automatically), i would argue the current use of empty is quite limited and i'd prefer to take this chance to make it a property as it is in pandas, since it is today too easy to misuse as expr.empty which is always truthy.

Contrarily the upgrade path from expr.empty() to expr.empty will always raise a clear Exception.

Update: Added the wrapper that @FabianHofmann sketched below to smooth the transition.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FabianHofmann]

I like that and wanted to do this for a long time, was scared of the breaking change though. could we instead define a callable Empty class returned by the property, which if called raises a deprecation warning? Something like

import warnings
import functools

class Empty:
    def __init__(self, value):
        self._value = bool(value)
    
    def __bool__(self):
        return self._value
    
    def __call__(self):
        warnings.warn(
            "Calling `.empty()` is deprecated, use `.empty` property instead",
            DeprecationWarning,
            stacklevel=2
        )
        return self._value
    

and then in the property

    @property
    def empty(self):
        is_empty = ...
        return Empty(is_empty)

[coroa]

I guess that would work, quite a bit of boilerplate for that simple change, but everyone loves downward compatibility!

[lkstrp]

We might also want to add __repr__ to the descriptor. Otherwise the memory reference is shown, which is confusing.

Even cleaner, which would not mock a simple boolean, the process would probably be to have a module wide option setting, raise a deprecation warning for .empty() without breaking behaviour, and allow opting in to use .empty.

[coroa]

Added the __repr__, but went with the wrapper. I am not a big fan of opt-in. I directly want to use the good shit.

[FabianHofmann]

great!