State keeps the vector field of an empty state variable

[michakraus]

What this changes

State keeps the vector field of an empty state variable.

_novectorfield(x) = ismissing(x) || all(ismissing, x) dropped a field whenever all(ismissing, x) held. Over a VectorfieldVariable — an AbstractArray — that is true exactly when some extent is zero, so a state variable with a zero extent lost its field. A 6 × 0 particle matrix then had no q̇ to fill, and integrate raised FieldError: type State has no field q̇ on any problem whose state is empty.

A field is now dropped only where _vectorfield returns missing, so a vector field mirrors the shape of its state variable, a zero extent included.

Why

An empty particle list is a valid configuration — a vacuum run. The change also removes a crash for every other empty state, and it makes the vectorfield NamedTuple's type inferable from the type of ics, where it was abstract.

Evidence

• all(ismissing, x) over a VectorfieldVariable: 6×0, 0×0, (0,), 0×6 and 3×0×2 give true; 6×3 and a 0-dimensional array give false.
• The new testset in test/data/state.jl asserts :q̇ ∈ keys(st) and size(st.q̇) == size(st.q) over 6×0, (0,), 3×0×2 and a 0-dimensional state, in Float32 and Float64.
• The suite passes with the change: States 92/92, State Variables 1738/1738, Aqua 10 pass and 1 broken, as on main.
• No non-empty caller changes: for a VectorfieldVariable with no zero extent and a non-missing element type, all(ismissing, x) is false, so the kept keys are the same.

Compatibility

A patch release. No non-empty result changes. The one visible difference is that keys(State(…)) of an empty state gains q̇, where before any integration of such a state failed.

The branch sets version = "0.15.1", which a dependant pins; the release commit closes out the CHANGELOG.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.20%. Comparing base (41c0816) to head (1d38e7a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   77.14%   77.20%   +0.05%     
==========================================
  Files          18       18              
  Lines         442      443       +1     
==========================================
+ Hits          341      342       +1     
  Misses        101      101              

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