api: patch empty size

Author: mloubout

devito/types/dimension.py

@@ -1025,7 +1025,7 @@ def _arg_defaults(self, _min=None, size=None, alias=None):
             if d1 < sympy.S.Infinity:
                 # We make sure the condition size matches the input size
                 size0 = (d1 - d0 + factor) // factor
-                if size < size0:
+                if size and size < size0:
                     raise ValueError(f"Incompatible size for ConditionalDimension "
                                      f"{self.name}: {size} < {size0}")
             else: