model: Fix initial value of LinePadding style (#405)

Author: palemieux

src/main/python/ttconv/style_properties.py

@@ -616,7 +616,7 @@ class LinePadding(StyleProperty):
 
     @staticmethod
     def make_initial_value():
-      return LengthType(value=1, units=LengthType.Units.c)
+      return LengthType(value=0, units=LengthType.Units.c)
 
     @staticmethod
     def validate(value: LengthType):