simplify check, add test

src/textual/_layout_resolve.py

@@ -56,7 +56,7 @@ def layout_resolve(total: int, edges: Sequence[Edge]) -> list[int]:
         ]
 
     _Fraction = Fraction
-    while None in sizes:
+    while flexible_edges:
         # Calculate number of characters in a ratio portion
         portion = _Fraction(
             remaining, sum((edge.fraction or 1) for _, edge in flexible_edges)

tests/test_layout_resolve.py

@@ -60,6 +60,7 @@ def test_two():
             [Edge(None, 4), Edge(None, 1), Edge(None, 1), Edge(None, 1)],
             [5, 2, 1, 2],
         ),
+        (2, [Edge(None, 1), Edge(None, 1), Edge(None, 1)], [1, 1, 1]),
     ],
 )
 def test_multiple(size, edges, result):