Add public per-slot emptiness check on expressions (`has_terms`) — PyPSA reaches into `.vars`/`_term` internals

[FBumann]

Motivation

PyPSA's nodal balance constraint needs to know which slots of an expression have no live variable terms, and currently has to reach into linopy internals to find out (pypsa/optimization/constraints.py#L1166):

lhs = merge(exprs, join="outer").reindex(name=buses)
...
empty_nodal_balance = (lhs.vars == -1).all("_term")

if empty_nodal_balance.any():
    if (empty_nodal_balance & (rhs != 0)).any().item():
        raise ValueError("Empty LHS with non-zero RHS in nodal balance constraint.")
    mask = ~empty_nodal_balance
else:
    mask = None

Buses with no attached components end up with all-dead terms (vars = -1) after the outer merge + reindex. PyPSA needs them to (a) raise on demand at a disconnected bus, and (b) mask those constraint rows out.

vars and "_term" are internal representation details. Downstream code depending on them couples PyPSA to linopy's storage layout, and shows a gap in the public API.

Why the existing API doesn't cover it

• expr.empty is a single bool for the whole expression.
• expr.isnull() is (vars == -1).all(helper_dims) & const.isnull() — absence (no terms and no constant). In the merge + reindex pattern above it happens to give the same answer (reindexed slots carry a NaN constant), but it tests a different concept: a slot whose constant has been filled — e.g. after fillna(0) — is no longer "null", yet still has no terms. The two concepts are also explicitly distinct in the v1 convention (feat: v1 semantic convention #717): dead terms at a present slot vs an absent slot.

So the check PyPSA actually means — "this constraint row has no variables" — has no public spelling.

Proposed API

@property
def has_terms(self) -> DataArray:
    """
    Boolean array, True at slots where the expression has at least one
    live term (vars != -1), reduced over the helper dims.
    """

on BaseExpression, so both LinearExpression and QuadraticExpression get it.

PyPSA's code then becomes internals-free:

has_terms = lhs.has_terms
if not has_terms.all():
    if (~has_terms & (rhs != 0)).any().item():
        raise ValueError("Empty LHS with non-zero RHS in nodal balance constraint.")
    mask = has_terms
else:
    mask = None

Relation to isnull()

has_terms and isnull() answer different questions and both have a place:

slot state	isnull()	~has_terms
live terms	False	False
no terms, const = NaN (absent, e.g. after reindex)	True	True
no terms, const = 0 (present slot, e.g. after fillna(0))	False	True

I'll follow up with a PR implementing this.