fix: Let Section.from_string handle None

These lines were removed---or somehow missing. This is probably not the
best way to do this, but it's expected by DRAGONS.

astrodata/utils.py

@@ -475,6 +475,9 @@ def from_shape(value):
     @staticmethod
     def from_string(value):
         """Produce a Section object from a string."""
+        if value is None:
+            return None
+
         return Section(
             *[
                 y