Add nullable strings

#19

Author: n1nj4t4nuk1

pyvalueobjects/strings/nullable_string.py

@@ -0,0 +1,10 @@
+from pyvalueobjects.abstract.nullablevalueobject import build_nullable
+from pyvalueobjects.strings.string import String
+
+_nullable_cls = build_nullable(String)
+
+
+class NullableString(_nullable_cls):
+
+    def __init__(self, value: str = None):
+        super().__init__(value)