fix: explicitly set outer join for expression merge

Set join="outer" when merging expressions without common coordinate
structure to ensure consistent behavior across xarray versions and
properly handle expressions with disjoint coordinates.

Author: FabianHofmann

linopy/expressions.py

@@ -1999,6 +1999,8 @@ def merge(
 
         if override:
             kwargs["join"] = "override"
+        else:
+            kwargs.setdefault("join", "outer")
 
     if dim == TERM_DIM:
         ds = xr.concat([d[["coeffs", "vars"]] for d in data], dim, **kwargs)