Fix ptex being included in all scies. (#124)

Only include it when needed.

The over-eager inclusion originated here in the 0.11.0 release:
#120 (comment)

CHANGES.md

@@ -1,5 +1,11 @@
 # Release Notes
 
+## 0.11.2
+
+This release fixes a bug introduced in the 0.11.0 release whereby a `ptex` binary was always
+included in each built scie even when the scie did not use `ptex`. This was functionally harmless
+but did bloat the size of the resulting scie binary by ~5MB.
+
 ## 0.11.1
 
 This release brings two fixes for `science` on Windows:

science/__init__.py

@@ -3,6 +3,6 @@
 
 from packaging.version import Version
 
-__version__ = "0.11.1"
+__version__ = "0.11.2"
 
 VERSION = Version(__version__)

science/dataclass/deserializer.py

@@ -41,6 +41,8 @@ def _parse_field(
                 ),
             ),
         )
+        if not data_value and default is None:
+            return cast(_F, None)
         return cast(_F, parse(data_value, dataclass_type, custom_parsers=custom_parsers))
 
     if type_.istype(int):