solidify on zero-weight self-neighbor as the encoding for islands

Author: ljwolf

libpysal/weights/weights.py

@@ -308,7 +308,7 @@ def to_adjlist(
         links = []
         for idx, neighb in self:
             if len(neighb) == 0:
-                links.append((idx, None, np.nan))
+                links.append((idx, idx, 0))
                 continue
             for n, w in neighb.items():
                 links.append((idx, n, w))