Allow crossing to return empty list if all values are masked

src/utils.py

@@ -23,6 +23,10 @@ def crossings(profile_x, profile_z, constant_z):
     profile_z = np.array(profile_z)[valid]
     profile_x = np.array(profile_x)[valid]
 
+    # Return empty list if mask removes all values
+    if profile_x.size == 0:
+        return []
+
     # Normalize the 'signal' to zero.
     # Use np.subtract rather than a list comprehension for performance reasons
     z = np.subtract(profile_z, constant_z)